diff --git a/notes/Leetcode 题解.md b/notes/Leetcode 题解.md index 5eea156a..1d12668e 100644 --- a/notes/Leetcode 题解.md +++ b/notes/Leetcode 题解.md @@ -3371,6 +3371,7 @@ class MyQueue { ```java class MyQueue { + private Stack in = new Stack(); private Stack out = new Stack(); @@ -3524,8 +3525,8 @@ public int[] dailyTemperatures(int[] temperatures) { int n = temperatures.length; int[] ret = new int[n]; Stack stack = new Stack<>(); - for(int i = 0; i < n; i++) { - while(!stack.isEmpty() && temperatures[i] > temperatures[stack.peek()]) { + for (int i = 0; i < n; i++) { + while (!stack.isEmpty() && temperatures[i] > temperatures[stack.peek()]) { int idx = stack.pop(); ret[idx] = i - idx; } @@ -3548,15 +3549,15 @@ Output: [-1,3,-1] public int[] nextGreaterElement(int[] nums1, int[] nums2) { Map map = new HashMap<>(); Stack stack = new Stack<>(); - for(int num : nums2){ - while(!stack.isEmpty() && num > stack.peek()){ + for (int num : nums2) { + while (!stack.isEmpty() && num > stack.peek()) { map.put(stack.pop(), num); } stack.add(num); } int[] ret = new int[nums1.length]; - for(int i = 0; i < nums1.length; i++){ - if(map.containsKey(nums1[i])) ret[i] = map.get(nums1[i]); + for (int i = 0; i < nums1.length; i++) { + if (map.containsKey(nums1[i])) ret[i] = map.get(nums1[i]); else ret[i] = -1; } return ret; @@ -3592,7 +3593,7 @@ Java 中的 **HashSet** 用于存储一个集合，并以 O(1) 的时间复杂 Java 中的 **HashMap** 主要用于映射关系，从而把两个元素联系起来。 -在对一个内容进行压缩或者其它转换时，利用 HashMap 可以把原始内容和转换后的内容联系起来。例如在一个简化 url 的系统中（[Leetcdoe : 535. Encode and Decode TinyURL (Medium)](https://leetcode.com/problems/encode-and-decode-tinyurl/description/)），利用 HashMap 就可以存储精简后的 url 到原始 url 的映射，使得不仅可以显示简化的 url，也可以根据简化的 url 得到原始 url 从而定位到正确的资源。 +在对一个内容进行压缩或者其它转换时，利用 HashMap 可以把原始内容和转换后的内容联系起来。例如在一个简化 url 的系统中[Leetcdoe : 535. Encode and Decode TinyURL (Medium)](https://leetcode.com/problems/encode-and-decode-tinyurl/description/)，利用 HashMap 就可以存储精简后的 url 到原始 url 的映射，使得不仅可以显示简化的 url，也可以根据简化的 url 得到原始 url 从而定位到正确的资源。 HashMap 也可以用来对元素进行计数统计，此时键为元素，值为计数。和 HashSet 类似，如果元素有穷并且范围不大，可以用整型数组来进行统计。 @@ -3602,13 +3603,13 @@ HashMap 也可以用来对元素进行计数统计，此时键为元素，值为 可以先对数组进行排序，然后使用双指针方法或者二分查找方法。这样做的时间复杂度为 O(NlogN)，空间复杂度为 O(1)。 -用 HashMap 存储数组元素和索引的映射，在访问到 nums[i] 时，判断 HashMap 中是否存在 target - nums[i] ，如果存在说明 target - nums[i] 所在的索引和 i 就是要找的两个数。该方法的时间复杂度为 O(N)，空间复杂度为 O(N)，使用空间来换取时间。 +用 HashMap 存储数组元素和索引的映射，在访问到 nums[i] 时，判断 HashMap 中是否存在 target - nums[i]，如果存在说明 target - nums[i] 所在的索引和 i 就是要找的两个数。该方法的时间复杂度为 O(N)，空间复杂度为 O(N)，使用空间来换取时间。 ```java public int[] twoSum(int[] nums, int target) { HashMap map = new HashMap<>(); - for(int i = 0; i < nums.length; i++){ - if(map.containsKey(target - nums[i])) return new int[]{map.get(target - nums[i]), i}; + for (int i = 0; i < nums.length; i++) { + if (map.containsKey(target - nums[i])) return new int[] { map.get(target - nums[i]), i }; else map.put(nums[i], i); } return null; @@ -3622,7 +3623,9 @@ public int[] twoSum(int[] nums, int target) { ```java public boolean containsDuplicate(int[] nums) { Set set = new HashSet<>(); - for (int num : nums) set.add(num); + for (int num : nums) { + set.add(num); + } return set.size() < nums.length; } ``` @@ -3657,13 +3660,15 @@ public int findLHS(int[] nums) { **最长连续序列** -[Leetcode : 128. Longest Consecutive Sequence (Medium)](https://leetcode.com/problems/longest-consecutive-sequence/description/) +[Leetcode : 128. Longest Consecutive Sequence (Hard)](https://leetcode.com/problems/longest-consecutive-sequence/description/) ```html Given [100, 4, 200, 1, 3, 2], The longest consecutive elements sequence is [1, 2, 3, 4]. Return its length: 4. ``` +题目要求：以 O(N) 的时间复杂度求解。 + ```java public int longestConsecutive(int[] nums) { Map numCnts = new HashMap<>(); @@ -3705,41 +3710,14 @@ s = "anagram", t = "nagaram", return true. s = "rat", t = "car", return false. ``` -字符串只包含小写字符，总共有 26 个小写字符。可以用 Hash Table 来映射字符与出现次数，因为键值范围很小，因此可以使用长度为 26 的整型数组对字符串出现的字符进行统计，比较两个字符串出现的字符数量是否相同。 +字符串只包含小写字符，总共有 26 个小写字符。可以用 Hash Table 来映射字符与出现次数，因为键值范围很小，因此可以使用长度为 26 的整型数组对字符串出现的字符进行统计，然后比较两个字符串出现的字符数量是否相同。 ```java public boolean isAnagram(String s, String t) { int[] cnts = new int[26]; - for(int i = 0; i < s.length(); i++) cnts[s.charAt(i) - 'a']++; - for(int i = 0; i < t.length(); i++) cnts[t.charAt(i) - 'a']--; - for(int i = 0; i < 26; i++) if(cnts[i] != 0) return false; - return true; -} -``` - -**字符串同构** - -[Leetcode : 205. Isomorphic Strings (Easy)](https://leetcode.com/problems/isomorphic-strings/description/) - -```html -Given "egg", "add", return true. -Given "foo", "bar", return false. -Given "paper", "title", return true. -``` - -记录一个字符上次出现的位置，如果两个字符串中某个字符上次出现的位置一样，那么就属于同构。 - -```java -public boolean isIsomorphic(String s, String t) { - int[] m1 = new int[256]; - int[] m2 = new int[256]; - for(int i = 0; i < s.length(); i++){ - if(m1[s.charAt(i)] != m2[t.charAt(i)]) { - return false; - } - m1[s.charAt(i)] = i + 1; - m2[t.charAt(i)] = i + 1; - } + for (char c : s.toCharArray()) cnts[c - 'a']++; + for (char c : t.toCharArray()) cnts[c - 'a']--; + for (int cnt : cnts) if (cnt != 0) return false; return true; } ``` @@ -3754,34 +3732,60 @@ Output : 7 Explanation : One longest palindrome that can be built is "dccaccd", whose length is 7. ``` -使用长度为 128 的整型数组来统计每个字符出现的个数，每个字符有偶数个可以用来构成回文字符串。因为回文字符串最中间的那个字符可以单独出现，所以如果有单独的字符就把它放到最中间。 +使用长度为 256 的整型数组来统计每个字符出现的个数，每个字符有偶数个可以用来构成回文字符串。因为回文字符串最中间的那个字符可以单独出现，所以如果有单独的字符就把它放到最中间。 ```java public int longestPalindrome(String s) { - int[] cnts = new int[128]; // ascii 码总共 128 个 - for(char c : s.toCharArray()) cnts[c]++; + int[] cnts = new int[256]; + for (char c : s.toCharArray()) cnts[c]++; int ret = 0; - for(int cnt : cnts) ret += (cnt / 2) * 2; - if(ret < s.length()) ret++; // 这个条件下 s 中一定有单个未使用的字符存在，可以把这个字符放到回文的最中间 + for (int cnt : cnts) ret += (cnt / 2) * 2; + if (ret < s.length()) ret++; // 这个条件下 s 中一定有单个未使用的字符存在，可以把这个字符放到回文的最中间 return ret; } ``` +**字符串同构** + +[Leetcode : 205. Isomorphic Strings (Easy)](https://leetcode.com/problems/isomorphic-strings/description/) + +```html +Given "egg", "add", return true. +Given "foo", "bar", return false. +Given "paper", "title", return true. +``` + +记录一个字符上次出现的位置，如果两个字符串中的字符上次出现的位置一样，那么就属于同构。 + +```java +public boolean isIsomorphic(String s, String t) { + int[] preIndexOfS = new int[256]; + int[] preIndexOfT = new int[256]; + for (int i = 0; i < s.length(); i++) { + char sc = s.charAt(i), tc = t.charAt(i); + if (preIndexOfS[sc] != preIndexOfT[tc]) return false; + preIndexOfS[sc] = i + 1; + preIndexOfT[tc] = i + 1; + } + return true; +} +``` + **判断一个整数是否是回文数** [Leetcode : 9. Palindrome Number (Easy)](https://leetcode.com/problems/palindrome-number/description/) -要求不能使用额外空间，也就不能将整数转换为字符串进行判断。 +题目要求：不能使用额外空间，也就不能将整数转换为字符串进行判断。 将整数分成左右两部分，右边那部分需要转置，然后判断这两部分是否相等。 ```java public boolean isPalindrome(int x) { - if(x == 0) return true; - if(x < 0) return false; - if(x % 10 == 0) return false; + if (x == 0) return true; + if (x < 0) return false; + if (x % 10 == 0) return false; int right = 0; - while(x > right){ + while (x > right) { right = right * 10 + x % 10; x /= 10; } @@ -3804,7 +3808,7 @@ Explanation: Six palindromic strings: "a", "a", "a", "aa", "aa", "aaa". ```java private int cnt = 0; public int countSubstrings(String s) { - for(int i = 0; i < s.length(); i++) { + for (int i = 0; i < s.length(); i++) { extendSubstrings(s, i, i); // 奇数长度 extendSubstrings(s, i, i + 1); // 偶数长度 } @@ -3812,7 +3816,7 @@ public int countSubstrings(String s) { } private void extendSubstrings(String s, int start, int end) { - while(start >= 0 && end < s.length() && s.charAt(start) == s.charAt(end)) { + while (start >= 0 && end < s.length() && s.charAt(start) == s.charAt(end)) { start--; end++; cnt++; @@ -3833,14 +3837,13 @@ Explanation: There are 6 substrings that have equal number of consecutive 1's an ```java public int countBinarySubstrings(String s) { int preLen = 0, curLen = 1, ret = 0; - for(int i = 1; i < s.length(); i++){ - if(s.charAt(i) == s.charAt(i-1)) curLen++; - else{ + for (int i = 1; i < s.length(); i++) { + if (s.charAt(i) == s.charAt(i-1)) curLen++; + else { preLen = curLen; curLen = 1; } - - if(preLen >= curLen) ret++; + if (preLen >= curLen) ret++; } return ret; } @@ -3848,7 +3851,7 @@ public int countBinarySubstrings(String s) { **字符串循环移位包含** -[ 编程之美：3.1](#) +[编程之美：3.1](#) ```html s1 = AABCD, s2 = CDAA @@ -3861,7 +3864,7 @@ s1 进行循环移位的结果是 s1s1 的子字符串，因此只要判断 s2 **字符串循环移位** -[ 编程之美：2.17](#) +[编程之美：2.17](#) 将字符串向右循环移动 k 位。 @@ -3900,12 +3903,12 @@ public void moveZeroes(int[] nums) { [Leetcode : 566. Reshape the Matrix (Easy)](https://leetcode.com/problems/reshape-the-matrix/description/) ```html -Input: -nums = +Input: +nums = [[1,2], [3,4]] r = 1, c = 4 -Output: +Output: [[1,2,3,4]] Explanation: The row-traversing of nums is [1,2,3,4]. The new reshaped matrix is a 1 * 4 matrix, fill it row by row by using the previous list. @@ -3933,19 +3936,191 @@ public int[][] matrixReshape(int[][] nums, int r, int c) { ```java public int findMaxConsecutiveOnes(int[] nums) { - int max = 0; - int cur = 0; + int max = 0, cur = 0; for (int num : nums) { - if (num == 0) cur = 0; - else { - cur++; - max = Math.max(max, cur); - } + cur = num == 0 ? 0 : cur + 1; + max = Math.max(max, cur); } return max; } ``` +**一个数组元素在 [1, n] 之间，其中一个数被替换为另一个数，找出丢失的数和重复的数** + +[Leetcode : 645. Set Mismatch (Easy)](https://leetcode.com/problems/set-mismatch/description/) + +```html +Input: nums = [1,2,2,4] +Output: [2,3] +``` + +```html +Input: nums = [1,2,2,4] +Output: [2,3] +``` + +最直接的方法是先对数组进行排序，这种方法时间复杂度为 O(NlogN)。本题可以以 O(N) 的时间复杂度、O(1) 空间复杂度来求解。 + +主要思想是通过交换数组元素，使得数组上的元素在正确的位置上。遍历数组，如果第 i 位上的元素不是 i + 1，那么就交换第 i 位和 nums[i] - 1 位上的元素，使得 num[i] - 1 位置上的元素为 nums[i]，也就是该位置上的元素是正确的。 + +```java +public int[] findErrorNums(int[] nums) { + for (int i = 0; i < nums.length; i++) { + while (nums[i] != i + 1) { + if (nums[i] == nums[nums[i] - 1]) { + return new int[]{nums[nums[i] - 1], i + 1}; + } + swap(nums, i, nums[i] - 1); + } + } + + return null; +} + +private void swap(int[] nums, int i, int j) { + int tmp = nums[i]; nums[i] = nums[j]; nums[j] = tmp; +} +``` + +类似题目： + +- [Leetcode :448. Find All Numbers Disappeared in an Array (Easy)](https://leetcode.com/problems/find-all-numbers-disappeared-in-an-array/description/)，寻找所有丢失的元素 +- [Leetcode : 442. Find All Duplicates in an Array (Medium)](https://leetcode.com/problems/find-all-duplicates-in-an-array/description/)，寻找所有重复的元素。 + +**找出数组中重复的数，数组值在 [1, n] 之间** + +[Leetcode : 287. Find the Duplicate Number (Medium)](https://leetcode.com/problems/find-the-duplicate-number/description/) + +要求不能修改数组，也不能使用额外的空间。 + +二分查找解法： + +```java +public int findDuplicate(int[] nums) { + int l = 1, h = nums.length - 1; + while (l <= h) { + int mid = l + (h - l) / 2; + int cnt = 0; + for (int i = 0; i < nums.length; i++) { + if (nums[i] <= mid) cnt++; + } + if (cnt > mid) h = mid - 1; + else l = mid + 1; + } + return l; +} +``` + +双指针解法，类似于有环链表中找出环的入口： + +```java +public int findDuplicate(int[] nums) { + int slow = nums[0], fast = nums[nums[0]]; + while (slow != fast) { + slow = nums[slow]; + fast = nums[nums[fast]]; + } + fast = 0; + while (slow != fast) { + slow = nums[slow]; + fast = nums[fast]; + } + return slow; +} +``` + +**有序矩阵查找** + +[Leetocde : 240. Search a 2D Matrix II (Medium)](https://leetcode.com/problems/search-a-2d-matrix-ii/description/) + +```html +[ + [ 1, 5, 9], + [10, 11, 13], + [12, 13, 15] +] +``` + +```java +public boolean searchMatrix(int[][] matrix, int target) { + if (matrix == null || matrix.length == 0 || matrix[0].length == 0) return false; + int m = matrix.length, n = matrix[0].length; + int row = 0, col = n - 1; + while (row < m && col >= 0) { + if (target == matrix[row][col]) return true; + else if (target < matrix[row][col]) col--; + else row++; + } + return false; +} +``` + +**有序矩阵的 Kth Element** + +[Leetcode : 378. Kth Smallest Element in a Sorted Matrix ((Medium))](https://leetcode.com/problems/kth-smallest-element-in-a-sorted-matrix/description/) + +```html +matrix = [ + [ 1, 5, 9], + [10, 11, 13], + [12, 13, 15] +], +k = 8, + +return 13. +``` + +解题参考：[Share my thoughts and Clean Java Code](https://leetcode.com/problems/kth-smallest-element-in-a-sorted-matrix/discuss/85173) + +二分查找解法： + +```java +public int kthSmallest(int[][] matrix, int k) { + int m = matrix.length, n = matrix[0].length; + int lo = matrix[0][0], hi = matrix[m - 1][n - 1]; + while(lo <= hi) { + int mid = lo + (hi - lo) / 2; + int cnt = 0; + for(int i = 0; i < m; i++) { + for(int j = 0; j < n && matrix[i][j] <= mid; j++) { + cnt++; + } + } + if(cnt < k) lo = mid + 1; + else hi = mid - 1; + } + return lo; +} +``` + +堆解法： + +```java +public int kthSmallest(int[][] matrix, int k) { + int m = matrix.length, n = matrix[0].length; + PriorityQueue pq = new PriorityQueue(); + for(int j = 0; j < n; j++) pq.offer(new Tuple(0, j, matrix[0][j])); + for(int i = 0; i < k - 1; i++) { // 小根堆，去掉 k - 1 个堆顶元素，此时堆顶元素就是第 k 的数 + Tuple t = pq.poll(); + if(t.x == m - 1) continue; + pq.offer(new Tuple(t.x + 1, t.y, matrix[t.x + 1][t.y])); + } + return pq.poll().val; +} + +class Tuple implements Comparable { + int x, y, val; + public Tuple(int x, int y, int val) { + this.x = x; this.y = y; this.val = val; + } + + @Override + public int compareTo(Tuple that) { + return this.val - that.val; + } +} +``` + **数组相邻差值的个数** [Leetcode : 667. Beautiful Arrangement II (Medium)](https://leetcode.com/problems/beautiful-arrangement-ii/description/) @@ -4112,185 +4287,6 @@ public int maxChunksToSorted(int[] arr) { } ``` - -**一个数组元素在 [1, n] 之间，其中一个数被替换为另一个数，找出丢失的数和重复的数** - -[Leetcode : 645. Set Mismatch (Easy)](https://leetcode.com/problems/set-mismatch/description/) - -```html -Input: nums = [1,2,2,4] -Output: [2,3] -``` - -```html -Input: nums = [1,2,2,4] -Output: [2,3] -``` - -最直接的方法是先对数组进行排序，这种方法时间复杂度为 O(nlogn)。本题可以以 O(n) 的时间复杂度、O(1) 空间复杂度来求解。 - -主要思想是通过交换数组元素，使得数组上的元素在正确的位置上。 - -遍历数组，如果第 i 位上的元素不是 i + 1 ，那么就交换第 i 位 和 nums[i] - 1 位上的元素，使得 num[i] - 1 的元素为 nums[i] ，也就是该位的元素是正确的。交换操作需要循环进行，因为一次交换没办法使得第 i 位上的元素是正确的。但是要交换的两个元素可能就是重复元素，那么循环就可能永远进行下去，终止循环的方法是加上 nums[i] != nums[nums[i] - 1 条件。 - -类似题目： - -- [Leetcode :448. Find All Numbers Disappeared in an Array (Easy)](https://leetcode.com/problems/find-all-numbers-disappeared-in-an-array/description/)，寻找所有丢失的元素 -- [Leetcode : 442. Find All Duplicates in an Array (Medium)](https://leetcode.com/problems/find-all-duplicates-in-an-array/description/)，寻找所有重复的元素。 - -```java -public int[] findErrorNums(int[] nums) { - for (int i = 0; i < nums.length; i++) { - while (nums[i] != i + 1) { - if (nums[i] == nums[nums[i] - 1]) { - return new int[]{nums[nums[i] - 1], i + 1}; - } - swap(nums, i, nums[i] - 1); - } - } - - return null; -} - -private void swap(int[] nums, int i, int j) { - int tmp = nums[i]; nums[i] = nums[j]; nums[j] = tmp; -} -``` - -**找出数组中重复的数，数组值在 [1, n] 之间** - -[Leetcode : 287. Find the Duplicate Number (Medium)](https://leetcode.com/problems/find-the-duplicate-number/description/) - -要求不能修改数组，也不能使用额外的空间。 - -二分查找解法： - -```java -public int findDuplicate(int[] nums) { - int l = 1, h = nums.length - 1; - while (l <= h) { - int mid = l + (h - l) / 2; - int cnt = 0; - for (int i = 0; i < nums.length; i++) { - if (nums[i] <= mid) cnt++; - } - if (cnt > mid) h = mid - 1; - else l = mid + 1; - } - return l; -} -``` - -双指针解法，类似于有环链表中找出环的入口： - -```java -public int findDuplicate(int[] nums) { - int slow = nums[0], fast = nums[nums[0]]; - while (slow != fast) { - slow = nums[slow]; - fast = nums[nums[fast]]; - } - fast = 0; - while (slow != fast) { - slow = nums[slow]; - fast = nums[fast]; - } - return slow; -} -``` - -**有序矩阵查找** - -[Leetocde : 240. Search a 2D Matrix II (Medium)](https://leetcode.com/problems/search-a-2d-matrix-ii/description/) - -```html -[ - [ 1, 5, 9], - [10, 11, 13], - [12, 13, 15] -] -``` - -```java -public boolean searchMatrix(int[][] matrix, int target) { - if (matrix == null || matrix.length == 0 || matrix[0].length == 0) return false; - int m = matrix.length, n = matrix[0].length; - int row = 0, col = n - 1; - while (row < m && col >= 0) { - if (target == matrix[row][col]) return true; - else if (target < matrix[row][col]) col--; - else row++; - } - return false; -} -``` - -**有序矩阵的 Kth Element** - -[Leetcode : 378. Kth Smallest Element in a Sorted Matrix ((Medium))](https://leetcode.com/problems/kth-smallest-element-in-a-sorted-matrix/description/) - -```html -matrix = [ - [ 1, 5, 9], - [10, 11, 13], - [12, 13, 15] -], -k = 8, - -return 13. -``` - -解题参考：[Share my thoughts and Clean Java Code](https://leetcode.com/problems/kth-smallest-element-in-a-sorted-matrix/discuss/85173) - -二分查找解法： - -```java -public int kthSmallest(int[][] matrix, int k) { - int m = matrix.length, n = matrix[0].length; - int lo = matrix[0][0], hi = matrix[m - 1][n - 1]; - while(lo <= hi) { - int mid = lo + (hi - lo) / 2; - int cnt = 0; - for(int i = 0; i < m; i++) { - for(int j = 0; j < n && matrix[i][j] <= mid; j++) { - cnt++; - } - } - if(cnt < k) lo = mid + 1; - else hi = mid - 1; - } - return lo; -} -``` - -堆解法： - -```java -public int kthSmallest(int[][] matrix, int k) { - int m = matrix.length, n = matrix[0].length; - PriorityQueue pq = new PriorityQueue(); - for(int j = 0; j < n; j++) pq.offer(new Tuple(0, j, matrix[0][j])); - for(int i = 0; i < k - 1; i++) { // 小根堆，去掉 k - 1 个堆顶元素，此时堆顶元素就是第 k 的数 - Tuple t = pq.poll(); - if(t.x == m - 1) continue; - pq.offer(new Tuple(t.x + 1, t.y, matrix[t.x + 1][t.y])); - } - return pq.poll().val; -} - -class Tuple implements Comparable { - int x, y, val; - public Tuple(int x, int y, int val) { - this.x = x; this.y = y; this.val = val; - } - - @Override - public int compareTo(Tuple that) { - return this.val - that.val; - } -} -``` - ## 链表 链表是空节点，或者有一个值和一个指向下一个链表的指针，因此很多链表问题可以用递归来处理。 @@ -4316,7 +4312,7 @@ B: b1 → b2 → b3 ```java public ListNode getIntersectionNode(ListNode headA, ListNode headB) { ListNode l1 = headA, l2 = headB; - while(l1 != l2){ + while (l1 != l2) { l1 = (l1 == null) ? headB : l1.next; l2 = (l2 == null) ? headA : l2.next; } @@ -4447,52 +4443,6 @@ public ListNode swapPairs(ListNode head) { } ``` -**根据有序链表构造平衡的 BST** - -[Leetcode : 109. Convert Sorted List to Binary Search Tree (Medium)](https://leetcode.com/problems/convert-sorted-list-to-binary-search-tree/description/) - -```html -Given the sorted linked list: [-10,-3,0,5,9], - -One possible answer is: [0,-3,9,-10,null,5], which represents the following height balanced BST: - - 0 - / \ - -3 9 - / / - -10 5 -``` - -```java -public TreeNode sortedListToBST(ListNode head) { - if (head == null) return null; - int size = size(head); - if (size == 1) return new TreeNode(head.val); - ListNode pre = head, mid = pre.next; - int step = 2; - while (step <= size / 2) { - pre = mid; - mid = mid.next; - step++; - } - pre.next = null; - TreeNode t = new TreeNode(mid.val); - t.left = sortedListToBST(head); - t.right = sortedListToBST(mid.next); - return t; -} - -private int size(ListNode node) { - int size = 0; - while (node != null) { - size++; - node = node.next; - } - return size; -} -``` - - **链表求和** [Leetcode : 445. Add Two Numbers II (Medium)](https://leetcode.com/problems/add-two-numbers-ii/description/) @@ -4532,46 +4482,6 @@ private Stack buildStack(ListNode l) { } ``` -**分隔链表** - -[Leetcode : 725. Split Linked List in Parts(Medium)](https://leetcode.com/problems/split-linked-list-in-parts/description/) - -```html -Input: -root = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10], k = 3 -Output: [[1, 2, 3, 4], [5, 6, 7], [8, 9, 10]] -Explanation: -The input has been split into consecutive parts with size difference at most 1, and earlier parts are a larger size than the later parts. -``` - -题目描述：把链表分隔成 k 部分，每部分的长度都应该尽可能相同，排在前面的长度应该大于等于后面的。 - -```java -public ListNode[] splitListToParts(ListNode root, int k) { - int N = 0; - ListNode cur = root; - while (cur != null) { - N++; - cur = cur.next; - } - int mod = N % k; - int size = N / k; - ListNode[] ret = new ListNode[k]; - cur = root; - for (int i = 0; cur != null && i < k; i++) { - ret[i] = cur; - int curSize = size + (mod-- > 0 ? 1 : 0); - for (int j = 0; j < curSize - 1; j++) { - cur = cur.next; - } - ListNode next = cur.next; - cur.next = null; - cur = next; - } - return ret; -} -``` - **回文链表** [Leetcode : 234. Palindrome Linked List (Easy)](https://leetcode.com/problems/palindrome-linked-list/description/) @@ -4652,6 +4562,46 @@ public ListNode oddEvenList(ListNode head) { } ``` +**分隔链表** + +[Leetcode : 725. Split Linked List in Parts(Medium)](https://leetcode.com/problems/split-linked-list-in-parts/description/) + +```html +Input: +root = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10], k = 3 +Output: [[1, 2, 3, 4], [5, 6, 7], [8, 9, 10]] +Explanation: +The input has been split into consecutive parts with size difference at most 1, and earlier parts are a larger size than the later parts. +``` + +题目描述：把链表分隔成 k 部分，每部分的长度都应该尽可能相同，排在前面的长度应该大于等于后面的。 + +```java +public ListNode[] splitListToParts(ListNode root, int k) { + int N = 0; + ListNode cur = root; + while (cur != null) { + N++; + cur = cur.next; + } + int mod = N % k; + int size = N / k; + ListNode[] ret = new ListNode[k]; + cur = root; + for (int i = 0; cur != null && i < k; i++) { + ret[i] = cur; + int curSize = size + (mod-- > 0 ? 1 : 0); + for (int j = 0; j < curSize - 1; j++) { + cur = cur.next; + } + ListNode next = cur.next; + cur.next = null; + cur = next; + } + return ret; +} +``` + ## 树 ### 递归 @@ -5545,6 +5495,52 @@ private void inOrder(TreeNode node, int k) { } ``` +**根据有序链表构造平衡的 BST** + +[Leetcode : 109. Convert Sorted List to Binary Search Tree (Medium)](https://leetcode.com/problems/convert-sorted-list-to-binary-search-tree/description/) + +```html +Given the sorted linked list: [-10,-3,0,5,9], + +One possible answer is: [0,-3,9,-10,null,5], which represents the following height balanced BST: + + 0 + / \ + -3 9 + / / + -10 5 +``` + +```java +public TreeNode sortedListToBST(ListNode head) { + if (head == null) return null; + int size = size(head); + if (size == 1) return new TreeNode(head.val); + ListNode pre = head, mid = pre.next; + int step = 2; + while (step <= size / 2) { + pre = mid; + mid = mid.next; + step++; + } + pre.next = null; + TreeNode t = new TreeNode(mid.val); + t.left = sortedListToBST(head); + t.right = sortedListToBST(mid.next); + return t; +} + +private int size(ListNode node) { + int size = 0; + while (node != null) { + size++; + node = node.next; + } + return size; +} +``` + + ### Trie