41. First Missing Positive(C++)

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.

 

Solution:

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