LeetCode题解：Next Permutation

Implement next permutation, which rearranges numbers into the lexicographically next greater permutation of numbers.

If such arrangement is not possible, it must rearrange it as the lowest possible order (ie, sorted in ascending order).

The replacement must be in-place, do not allocate extra memory.

Here are some examples. Inputs are in the left-hand column and its corresponding outputs are in the right-hand column.
1,2,3 → 1,3,2
3,2,1 → 1,2,3
1,1,5 → 1,5,1

题意：求给定数字的其他组合，要求组成树最大，如果已经是最大，则降序排列

解决思路：先找到非降序排列，根据性质将小于它的最小数交换

代码：

public class Solution {
    public void nextPermutation(int[] nums) {
        if(nums == null || nums.length < 2){
            return;
        }

        int index = nums.length - 2;
        for(; index >= 0 && nums[index + 1] <= nums[index]; index--) ;

        if(index >= 0){
            int j = index + 1;
            for(; j < nums.length && nums[index] < nums[j]; j++) ;
            swap(nums, index, j - 1);
        }

        index++;
        int k = nums.length - 1;
        for(; index < k; index++, k--){
            swap(nums, index, k);
        }
    }

    private void swap(int[] nums, int start, int end){
        int temp = nums[start];
        nums[start] = nums[end];
        nums[end] = temp;
    }
}