leetcode 41. First Missing Positive

Given an unsorted integer array, find the smallest missing positive integer.

Example 1:

Input: [1,2,0]
Output: 3
Example 2:

Input: [3,4,-1,1]
Output: 2
Example 3:

Input: [7,8,9,11,12]
Output: 1
Note:

Your algorithm should run in O(n) time and uses constant extra space.

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