first-missing-positive.cpp

class Solution {//The Pigeonhole Principle
public:
    int firstMissingPositive(int A[], int n) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        for(int i = 0; i < n; ++i) {
            while(A[i] <= n && A[i] > 0 && A[i] != A[A[i]-1]) {
                swap(A[i], A[A[i] - 1]);
            }
        }
        for(int i = 0; i < n; ++i) {
            if(A[i] != i + 1)
                return i + 1;
        }
        return n + 1;
    }
};