Added solution for single number iii (#379)

Author: Xtha-Sunil

C++/single-number-iii.cpp

@@ -0,0 +1,22 @@
+class Solution {
+public:
+    vector<int> singleNumber(vector<int>& nums) {
+        long long allxor;
+        for(int x:nums){
+            allxor^=x;
+        }
+
+        int mask =  allxor &(-allxor);
+        int a=0;
+        int b=0;
+        for(int x:nums){
+            if(mask&x){
+                a^=x;
+            }
+            else{
+                b^=x;
+            }
+        }
+        return {a,b};
+    }
+};