LeetCode題解：Next Permutation

LeetCode題解：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

題意：求給定數字的其他組合，要求組成樹最大，如果已經是最大，則降序排列

解決思路：先找到非降序排列，根據性質將小於它的最小數交換

代碼：

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;
    }
}