leetcode——41——First Missing Positive

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

For example,
Given [1,2,0] return 3,
and [3,4,-1,1] return 2.

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

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

    }
};