## leetcode - 31. Next Permutation - Medium

### descrition

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``````

## 解析

### 方法 2

• 在 a[i,...n-1] 是非递增有序（递减有序）的数组，因为 for 循环提前结束的条件是 a[i-1] < a[i]，如果没有提前结束则说明，a[i-1] > a[i]。在这样的情况下，我们需要找到 a[i,...,n-1] 中比 a[i-1] 大的数中最小的那个，假设为 a[j]。将 a[j] 和 a[i-1] 交换，由数组的性质可得 a[i,...,n-1] 依然保持非递增（递减）有序，这时我们只需要将 a[i,...,n-1] 反转，即可得到下一个最小的排列。
• 如果循环没有提前结束，则说明每一次比较都是 a[i-1] > a[i]，说明整个数组非递增（递减）有序。在这样的情况下直接将数组反转即可得到最小的排列。

## code

``````
#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

class Solution{
public:
void nextPermutation(vector<int>& nums) {
if(nums.empty())
return;

int isplit = -1;
for(int i=nums.size()-1; i>0; i--){
if(nums[i-1] < nums[i]){
isplit = i-1;
break;
}
}

// if isplit == -1, which indicate nums is in descending
// otherwhise, [isplit+1, n-1] is descending

if(isplit == -1){
reverse(nums, 0, nums.size()-1);
}else{
// construct next permutation
// find the smallest one from the numbers which larger than nums[isplit]
int ismallest = nums.size()-1;
while(ismallest>isplit && nums[ismallest] <= nums[isplit])
ismallest--;

// nums[ismallest] > nums[isplit] >= nums[ismallest+1, ... , n-1]
// and  nums[isplit+1, ismallest-1] >= nums[ismallest] >= nums[ismallest+1,..,n-1]
swap(nums[isplit], nums[ismallest]);

// nums[isplit+1, ... , n-1] still in descending
// so just reverse
reverse(nums, isplit+1, nums.size()-1);
}
}

void reverse(vector<int>& nums, int ileft, int iright){
while(ileft < iright){
swap(nums[ileft], nums[iright]);
ileft++;
iright--;
}
}
};

int main()
{
return 0;
}``````

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