41. First Missing Positive

Given an unsorted integer array nums, return the smallest missing positive integer.

You must implement an algorithm that runs in O(n) time and uses constant extra space.

class Solution {
    bool check[500002] {false,};
public:
    int firstMissingPositive(vector<int>& nums) {
        int res = 1;
        for(auto& n : nums) {
            if(1 <= n and n <= 500000) check[n] = true;
            while(check[res]) res++;
        }
        return res;
    }
};