added FindTheDuplicateNumber

algorithms/cpp/findTheDuplicateNumber/findTheDuplicateNumber.cpp

@@ -0,0 +1,44 @@
+// Source : https://leetcode.com/problems/find-the-duplicate-number/
+// Author : Calinescu Valentin
+// Date   : 2015-10-19
+
+/*************************************************************************************** 
+ *
+ * Given an array nums containing n + 1 integers where each integer is between 1 and 
+ * n (inclusive), prove that at least one duplicate number must exist.
+ * Assume that there is only one duplicate number, find the duplicate one.
+ *
+ * Note:
+ * You must not modify the array (assume the array is read only).
+ * You must use only constant, O(1) extra space.
+ * Your runtime complexity should be less than O(n2).
+ * There is only one duplicate number in the array, but it could be repeated more than
+ * once.
+ * Credits:
+ * Special thanks to @jianchao.li.fighter for adding this problem and creating all test
+ * cases.
+ *               
+ ***************************************************************************************/
+
+
+
+/* 
+ * Solutions
+ * =========
+ *
+ * A simple solution would be to sort the array and then look for equal consecutive elements.
+ * 
+ * Time Complexity: O(N log N)
+ * Space Complexity: O(1)
+ * 
+ */
+#include <algorithm>
+class Solution {
+public:
+    int findDuplicate(vector<int>& nums) {
+        sort(nums.begin(), nums.end());
+        for(vector<int>::iterator it = nums.begin(); it != nums.end(); ++it)
+            if(*it == *(it + 1))
+                return *it;
+    }
+};