Given an unsorted integer array, find the first missing positive integer. Example: Given [1,2,0] return 3, [3,4,-1,1] return 2, [-8, -7, -6] returns 1 Your algorithm should run in O(n) time and use constant space. Tags: InterviewBit Array Problem https://www.interviewbit.com/problems/first-missing-integer/

FindMissingInteger.cpp

int Solution::firstMissingPositive(vector<int> &A) {
    sort(A.begin(), A.end());
    int c = 1;
    for (auto const& a : A)
    {
        if (a<1)
            continue;
        else if (a==c)
        {
            ++c;
            continue;
        }
        else
            break;
    }
    return c;
}

Solution must be O(n) and your solution in O(nlogn)