# Problem 31: Next Permutation

> <https://leetcode.com/problems/next-permutation/>

## 思路

![](https://1241747088-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-Lpv9LvBSlFaukf_ALqh%2F-Lpv9NPw3Ji1X5Vk8CES%2F-Lpv9vthXIn2h2XCcfY9%2FnextPermutation.png?generation=1569729533114305\&alt=media)

```java
public class Solution {
    public void nextPermutation(int[] nums) {
        if (nums == null || nums.length == 0) {
            return;
        }

        int len = nums.length;
        for (int i = len - 2; i >= 0; i--) {
            if (nums[i + 1] > nums[i]) {
                int j;
                for (j = len - 1; j > i; j--) {
                    if (nums[j] > nums[i]) {
                        break;
                    }
                }
                swap(nums, i, j);
                reverse(nums, i + 1, len - 1);
                return;
            }

        }
        reverse(nums, 0, len - 1);
    }

    private void swap(int[] nums, int i, int j) {
        int temp = nums[i];
        nums[i] = nums[j];
        nums[j] = temp;
    }

    private void reverse(int[] nums, int start, int end) {
        for (int i = start, j = end; i < j; i++, j--) {
            swap(nums, i, j);
        }
    }

}
```

## 易错点

1. 这两个 swap() 和　reverse() 的函数一定要熟练掌握！
2. index 很容易搞错，什么时候是 len - 2，什么时候是 len - 1
3. 如果 nums\[i + 1] > nums\[i] 发生在中间的时候，立即 swap，reverse；如果一直到退出循环都遇到，那就是发生了形如 “321” 的情况，直接reverse 就好了


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