# 1053. Previous Permutation With One Swap
###### tags: `Leetcode` `Medium` `FaceBook` `Permutation`
Link: https://leetcode.com/problems/previous-permutation-with-one-swap/
## 思路
和[0031. Next Permutation](https://hackmd.io/TsbCCO9UTJK23dz4ewwVbQ)有点像，但又不完全一样
通过观察可以发现，这题要找从右往左第一个升序的也就是arr[i]>arr[i+1]，然后把arr[i]和后面的数字里最大的但小于arr[i]的数字和arr[i]交换
## Code

class Solution {
    public int[] prevPermOpt1(int[] arr) {
        int largerIdx = -1;
        for(int i = arr.length-2;i >= 0;i--){
            if(arr[i+1] < arr[i]){
                largerIdx = i;
                break;
            }
        }
        if(largerIdx == -1) return arr;
        int swapIdx = largerIdx+1;
        for(int i = largerIdx+1;i < arr.length;i++){
            if(arr[i]<arr[largerIdx] && arr[i]>arr[swapIdx]){
                swapIdx = i;
            }
        }
        swap(arr, largerIdx, swapIdx);
        return arr;
    }
    public void swap(int[] arr, int a, int b){
        int temp = arr[a];
        arr[a] = arr[b];
        arr[b] = temp;
    }
}