add firstMissingPositive.cpp

Author: kamyu104

C++/firstMissingPositive.cpp

@@ -0,0 +1,21 @@
+// Time Complexity: O(n)
+// Space Complexity: O(1)
+
+class Solution {
+    public:
+        int firstMissingPositive(int A[], int n) {
+            int i;
+            bucketSort(A, n);
+            for(i = 0; i < n && A[i] == i + 1; ++i);
+            return i + 1;
+        }
+
+    private:
+        void bucketSort(int A[], int n) {
+            for(int i = 0; i < n; ++i) {
+                for (; A[i] != i + 1 && A[i] > 0 && A[i] <= n && A[i] != A[A[i] - 1];) {
+                    swap(A[i], A[A[i] - 1]);
+                }
+            }
+        }
+};