First Missing Positive

Desicription

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

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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[i] != nums[nums[i] - 1])
swap(nums[i], nums[nums[i] - 1]);
for(int i = 0; i < n; i++)
if(nums[i] != i + 1) return i + 1;
return n + 1;
}
};