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) {
int i = 0;
int size = nums.size();
while (i < size) {
if(nums[i] == i + 1)
i++;
else if(nums[i] < i+1 || nums[i] > size || nums[i] == nums[nums[i]-1] ){
nums[i] = nums[--size];
}else{
swap(nums[nums[i]-1],nums[i]);
}
}
return i+1;
}
};