[A] the arrangement LeetCode 31

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【answer】

From right to left to find the position of a first drop I (i.e. satisfies nums [i] <nums [i + 1]);
then [i + 1..len-1] found inside this interval a maximum subscript k , so that the nums [K]> the nums I
note that the nature of a (i + 1..len-1) this paragraph is a monotonically decreasing
and swap (nums [k], nums [i]);
and then [i + 1 ..len-1] this stretch of flipping it.
You can get a next_permutation

[Code]

class Solution {
public:
    void swap(int &a,int &b){
        int t = a;a = b;b = t;
    }

    void _reverse(vector<int>&nums,int l,int r){
        while(l<=r){
            swap(nums[l],nums[r]);
            l++;r--;
        }
    }

    void nextPermutation(vector<int>& nums) {
        int len = nums.size();
        int j = -1;
        for (int i = len-1;i >= 0;i--){
            if (i-1>=0 && nums[i]>nums[i-1]){
                j = i-1;
                break;
            }
        }
        if (j==-1){
            _reverse(nums,0,len-1);   
        }else{
            for (int i = len-1;i >= 0;i--){
                if (nums[i]>nums[j]){
                    swap(nums[i],nums[j]);
                    break;
                }
            }
            _reverse(nums,j+1,len-1);
        }
    }
};