feat(cpp): add lonely_integer.cpp

Author: ongshunping

cpp/Bit Manipulation/lonely_integer.cpp

@@ -0,0 +1,13 @@
+#include <vector>
+
+int lonelyInteger(std::vector<int>& nums) {
+    int res = 0;
+    // XOR each element of the array so that duplicate values will
+    // cancel each other out (x ^ x == 0).
+    for (int i = 0; i < nums.size(); i++) {
+        res ^= nums[i];
+    }
+    // 'res' will store the lonely integer because it would not have
+    // been canceled out by any duplicate.
+    return res;
+}