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CyC2018
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* [哈希表](#哈希表)
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* [字符串](#字符串)
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* [数组与矩阵](#数组与矩阵)
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* [1-n 分布](#1-n-分布)
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* [有序矩阵](#有序矩阵)
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* [链表](#链表)
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* [树](#树)
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* [递归](#递归)
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@ -730,6 +728,51 @@ public List<Integer> topKFrequent(int[] nums, int k) {
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}
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```
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**按照字符出现次数对字符串排序**
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[Leetcode : 451. Sort Characters By Frequency (Medium)](https://leetcode.com/problems/sort-characters-by-frequency/description/)
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```html
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Input:
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"tree"
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Output:
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"eert"
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Explanation:
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'e' appears twice while 'r' and 't' both appear once.
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So 'e' must appear before both 'r' and 't'. Therefore "eetr" is also a valid answer.
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```
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```java
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public String frequencySort(String s) {
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Map<Character, Integer> map = new HashMap<>();
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for (char c : s.toCharArray()) {
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map.put(c, map.getOrDefault(c, 0) + 1);
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}
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List<Character>[] frequencyBucket = new List[s.length() + 1];
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for(char c : map.keySet()){
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int f = map.get(c);
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if (frequencyBucket[f] == null) {
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frequencyBucket[f] = new ArrayList<>();
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}
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frequencyBucket[f].add(c);
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}
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StringBuilder str = new StringBuilder();
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for (int i = frequencyBucket.length - 1; i >= 0; i--) {
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if (frequencyBucket[i] == null) {
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continue;
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}
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for (char c : frequencyBucket[i]) {
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for (int j = 0; j < i; j++) {
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str.append(c);
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}
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}
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}
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return str.toString();
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}
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```
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## 搜索
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深度优先搜索和广度优先搜索广泛运用于树和图中，但是它们的应用远远不止如此。
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@ -3506,6 +3549,45 @@ public int findLHS(int[] nums) {
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}
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```
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**最长连续序列**
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[Leetcode : 128. Longest Consecutive Sequence (Medium)](https://leetcode.com/problems/longest-consecutive-sequence/description/)
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```html
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Given [100, 4, 200, 1, 3, 2],
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The longest consecutive elements sequence is [1, 2, 3, 4]. Return its length: 4.
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```
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```java
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public int longestConsecutive(int[] nums) {
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Map<Integer, Integer> numCnts = new HashMap<>();
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for (int num : nums) {
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numCnts.put(num, 1);
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}
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for (int num : nums) {
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count(numCnts, num);
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}
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int max = 0;
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for (int num : nums) {
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max = Math.max(max, numCnts.get(num));
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}
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return max;
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}
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private int count(Map<Integer, Integer> numCnts, int num) {
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if (!numCnts.containsKey(num)) {
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return 0;
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}
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int cnt = numCnts.get(num);
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if (cnt > 1) {
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return cnt;
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}
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cnt = count(numCnts, num + 1) + 1;
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numCnts.put(num, cnt);
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return cnt;
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}
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```
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## 字符串
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**两个字符串包含的字符是否完全相同**
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@ -3707,7 +3789,105 @@ public void moveZeroes(int[] nums) {
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}
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```
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### 1-n 分布
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**对角元素相等的矩阵**
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[Leetcode : 766. Toeplitz Matrix (Easy)](https://leetcode.com/problems/toeplitz-matrix/description/)
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```html
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1234
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5123
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9512
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In the above grid, the diagonals are "[9]", "[5, 5]", "[1, 1, 1]", "[2, 2, 2]", "[3, 3]", "[4]", and in each diagonal all elements are the same, so the answer is True.
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```
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```java
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public boolean isToeplitzMatrix(int[][] matrix) {
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for (int i = 0; i < matrix[0].length; i++) {
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if (!check(matrix, matrix[0][i], 0, i)) {
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return false;
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}
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}
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for (int i = 0; i < matrix.length; i++) {
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if (!check(matrix, matrix[i][0], i, 0)) {
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return false;
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}
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}
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return true;
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}
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private boolean check(int[][] matrix, int expectValue, int row, int col) {
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if (row >= matrix.length || col >= matrix[0].length) {
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return true;
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}
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if (matrix[row][col] != expectValue) {
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return false;
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}
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return check(matrix, expectValue, row + 1, col + 1);
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}
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```
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**嵌套数组**
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[Leetcode : 565. Array Nesting (Medium)](https://leetcode.com/problems/array-nesting/description/)
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```html
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Input: A = [5,4,0,3,1,6,2]
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Output: 4
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Explanation:
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A[0] = 5, A[1] = 4, A[2] = 0, A[3] = 3, A[4] = 1, A[5] = 6, A[6] = 2.
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One of the longest S[K]:
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S[0] = {A[0], A[5], A[6], A[2]} = {5, 6, 2, 0}
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```
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题目描述：S[i] 表示一个集合，集合的第一个元素是 A[i]，第二个元素是 A[A[i]]，如此嵌套下去。求最大的 S[i]。
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```java
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public int arrayNesting(int[] nums) {
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int max = 0;
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for (int i = 0; i < nums.length; i++) {
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int cnt = 0;
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for (int j = i; nums[j] != -1; ) {
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cnt++;
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int t = nums[j];
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nums[j] = -1; // 标记该位置已经被访问
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j = t;
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}
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max = Math.max(max, cnt);
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}
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return max;
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}
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```
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**分隔数组**
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[Leetcode : 769. Max Chunks To Make Sorted (Medium)](https://leetcode.com/problems/max-chunks-to-make-sorted/description/)
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```html
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Input: arr = [1,0,2,3,4]
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Output: 4
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Explanation:
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We can split into two chunks, such as [1, 0], [2, 3, 4].
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However, splitting into [1, 0], [2], [3], [4] is the highest number of chunks possible.
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```
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题目描述：分隔数组，使得对每部分排序后数组就为有序。
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```java
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public int maxChunksToSorted(int[] arr) {
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if (arr == null) return 0;
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int ret = 0;
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int right = arr[0];
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for (int i = 0; i < arr.length; i++) {
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right = Math.max(right, arr[i]);
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if (right == i) ret++;
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}
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return ret;
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}
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```
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**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**
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@ -3795,9 +3975,9 @@ public int findDuplicate(int[] nums) {
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}
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```
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### 有序矩阵
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**有序矩阵查找**
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有序矩阵指的是行和列分别有序的矩阵。一般可以利用有序性使用二分查找方法。
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[Leetocde : 240. Search a 2D Matrix II (Medium)](https://leetcode.com/problems/search-a-2d-matrix-ii/description/)
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```html
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[
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@ -3807,10 +3987,6 @@ public int findDuplicate(int[] nums) {
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]
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```
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**有序矩阵查找**
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[Leetocde : 240. Search a 2D Matrix II (Medium)](https://leetcode.com/problems/search-a-2d-matrix-ii/description/)
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```java
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public boolean searchMatrix(int[][] matrix, int target) {
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if (matrix == null || matrix.length == 0 || matrix[0].length == 0) return false;
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@ -5063,6 +5239,66 @@ class MapSum {
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## 图
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**冗余连接**
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[Leetcode : 684. Redundant Connection (Medium)](https://leetcode.com/problems/redundant-connection/description/)
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```html
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Input: [[1,2], [1,3], [2,3]]
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Output: [2,3]
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Explanation: The given undirected graph will be like this:
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1
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/ \
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2 - 3
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```
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题目描述：有一系列的边连成的图，找出一条边，移除它之后该图能够成为一棵树。
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使用 Union-Find。
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```java
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public int[] findRedundantConnection(int[][] edges) {
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int N = edges.length;
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UF uf = new UF(N);
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for (int[] e : edges) {
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int u = e[0], v = e[1];
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if (uf.find(u) == uf.find(v)) {
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return e;
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}
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uf.union(u, v);
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}
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return new int[]{-1, -1};
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}
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private class UF {
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int[] id;
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UF(int N) {
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id = new int[N + 1];
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for (int i = 0; i < id.length; i++) {
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id[i] = i;
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}
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}
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void union(int u, int v) {
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int uID = find(u);
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int vID = find(v);
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if (uID == vID) {
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return;
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}
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for (int i = 0; i < id.length; i++) {
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if (id[i] == uID) {
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id[i] = vID;
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}
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}
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}
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int find(int p) {
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return id[p];
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}
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}
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```
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## 位运算
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**1. 基本原理**
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