diff --git a/Sets/Set Mismatch b/Sets/Set Mismatch
new file mode 100644
index 0000000..38f84d0
--- /dev/null
+++ b/Sets/Set Mismatch	
@@ -0,0 +1,12 @@
+The idea is using array indexing, that is putting each nums[i] into the position with index nums[i] - 1. Then, the array becomes [1,2,3,4,5...,n]. So we can find the duplicate number when nums[i] != i+1.
+
+```
+vector<int> findErrorNums(vector<int>& nums) {
+        for(int i = 0; i<nums.size(); i++){
+            while(nums[i] != nums[nums[i] - 1])swap(nums[i], nums[nums[i] - 1]);
+        }
+        for(int i = 0; i<nums.size() ; i++){
+            if(nums[i] != i + 1)return {nums[i], i + 1};
+        }
+    }
+    ````