finding first missing positive

firstmissingpositive.cpp

@@ -0,0 +1,21 @@
+/*
+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
+{
+public:
+    int firstMissingPositive(int A[], int n)
+    {
+        for(int i = 0; i < n; ++ i)
+            while(A[i] > 0 && A[i] <= n && A[A[i] - 1] != A[i])
+                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;
+    }
+};