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