Update and rename Missing number to Missing number.cpp

Author: Gauranshgoel123

Missing number

@@ -0,0 +1,9 @@
+class Solution {
+public:
+    int missingNumber(vector<int>& nums) {
+        int n = nums.size() + 1;
+        int expectedSum = (n * (n - 1)) / 2;
+        for (int n : nums) expectedSum -= n;
+        return expectedSum;
+    }
+};