while (nums[i] != i + 1 && nums[nums[i] - 1] != nums[i]) {
swap(nums, i, nums[i] - 1);
}
}
for (int i = 0; i <nums.length;i++){
if (nums[i] != i + 1) {
return new int[]{nums[i], 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：
- [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
- [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。
# 7. 找出数组中重复的数，数组值在 [1, n] 之间
[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;
}
```
# 8. 数组相邻差值的个数
[667. Beautiful Arrangement II (Medium)](https://leetcode.com/problems/beautiful-arrangement-ii/description/)
```html
Input: n = 3, k = 2
Output: [1, 3, 2]
Explanation: The [1, 3, 2] has three different positive integers ranging from 1 to 3, and the [2, 1] has exactly 2 distinct integers: 1 and 2.
```
题目描述：数组元素为 1\~n 的整数，要求构建数组，使得相邻元素的差值不相同的个数为 k。
让前 k+1 个元素构建出 k 个不相同的差值，序列为：1 k+1 2 k 3 k-1 ... k/2 k/2+1.
```java
public int[] constructArray(int n, int k) {
int[] ret = new int[n];
ret[0] = 1;
for (int i = 1, interval = k; i <= k; i++, interval--) {
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.
```
```java
public boolean isToeplitzMatrix(int[][] matrix) {
for (int i = 0; i <matrix[0].length;i++){
if (!check(matrix, matrix[0][i], 0, i)) {
return false;
}
}
for (int i = 0; i <matrix.length;i++){
if (!check(matrix, matrix[i][0], i, 0)) {
return false;
}
}
return true;
}
private boolean check(int[][] matrix, int expectValue, int row, int col) {
if (row >= matrix.length || col >= matrix[0].length) {
return true;
}
if (matrix[row][col] != expectValue) {
return false;
}
return check(matrix, expectValue, row + 1, col + 1);
