第一个缺失的正整数

class Solution {public:    int firstMissingPositive(vector<int>& nums) {        int n = nums.size();        int i = 0;        for( ;i<n;){            if(nums[i] == i+1){                i++;            }else if(nums[i] <= 0 || nums[i] > n || nums[i] == nums[nums[i]-1]){                swap(nums[i],nums[n-1]);                --n;            } else {                swap(nums[i],nums[nums[i]-1]);            }        }        return i+1;    }};

https://leetcode.com/problems/first-missing-positive/#/description