@@ -0,0 +1,28 @@
+#include<vector>
+#include<algorithm>
+#include<iostream>
+using namespace std;
+
+class Solution {
+public:
+int missingNumber(vector<int>& nums) {
+const int len = nums.size(); // len = n-1
+int expectSum = len*(len+1)/2; // (n-1)*n/2
+int sum = 0;
+for (int i = 0; i < len; i++)
+sum += nums[i];
+
+return (expectSum - sum);
+}
+};
+
+// Test
+int main()
+{
+Solution sol;
+vector<int> nums = {9, 6, 4, 2, 3, 5, 7, 0, 1};
+auto res = sol.missingNumber(nums);
+cout << res << endl;
+
+return 0;
+}