diff --git a/2018-feb/adhoc-thinking/src/com/alg/top20/adhoc/TwoSum.cpp b/2018-feb/adhoc-thinking/src/com/alg/top20/adhoc/TwoSum.cpp
new file mode 100644
index 0000000..3d6b3a3
--- /dev/null
+++ b/2018-feb/adhoc-thinking/src/com/alg/top20/adhoc/TwoSum.cpp
@@ -0,0 +1,55 @@
+#include <iostream>
+#include <algorithm>
+#include <set>
+using namespace std;
+
+// sort and check adjacent element 
+int findDuplicate1(int arr[],int n){
+	sort(arr,arr+n);
+	for(int i=1;i<n;++i){
+		if(arr[i]==arr[i-1]){
+			return arr[i];
+		}
+	}
+	return -1;
+}
+
+//in place check
+int findDuplicate2(int arr[],int n){
+		
+	for(int i=0;i<n;i++){
+		if(arr[abs(arr[i])] < 0)
+		      return abs(arr[i]);
+		else       
+		     arr[abs(arr[i])]*=-1;
+	}
+	return 0;
+}
+//using set data structure
+int findDuplicate3(int arr[],int n){
+	set<int> st;
+	st.insert(arr[0]);
+	for(int i=1;i<n;i++){
+		if(st.find(arr[i])!=st.end())
+		        return arr[i];
+		st.insert(arr[i]);
+	}
+}
+
+int main(){
+	int n;
+	cin>>n;
+	int arr[n];
+	int i;
+	
+	// generating a test case
+	for(i=0;i<n-1;i++){
+		arr[i]=n-i-1;
+	}
+	arr[i]=n-1; //making last element duplicate
+	
+	cout<<findDuplicate1(arr,n);
+	cout<<findDuplicate2(arr,n);
+	cout<<findDuplicate3(arr,n);
+	
+}