First_Missing_Positive.cpp

class Solution {
  public:
    int firstMissingPositive(int A[], int n) {
        int i = 0;
        while (i < n)  {
            if (A[i] != (i + 1) && A[i] > 0 && A[i] <= n && A[A[i] - 1] != A[i]) {
                swap(A[i], A[A[i] - 1]);
            } else {
                i++;
            }
        }
        for (i = 0; i < n; ++i) {
            if (A[i] != (i + 1)) {
                return i + 1;
            }
        }
        return n + 1;
    }
};