diff --git a/124-B2/SocialNetworkHard.cpp b/124-B2/SocialNetworkHard.cpp
new file mode 100644
index 0000000..6263906
--- /dev/null
+++ b/124-B2/SocialNetworkHard.cpp
@@ -0,0 +1,45 @@
+    #include <bits/stdc++.h>
+    #define ll long long
+     
+    using namespace std;
+     
+    int main(void)
+    {
+    	ll int n, k;
+    	ll int id;
+    	cin>>n>>k;
+    	set<ll int> s;
+    	queue<ll int> q;
+     
+    	for(ll int i=0; i<n; i++)
+    	{
+    		cin>>id;
+    		if(!s.count(id)){
+    			if(q.size()>=k)
+    			{
+    				ll int cur = q.front();
+    				q.pop();
+    				s.erase(cur);
+    			}
+    			s.insert(id);
+    			q.push(id);
+    		}
+    	}
+     
+    	vector<ll int> res;
+    	while(!q.empty())
+    	{
+    		res.push_back(q.front());
+    		q.pop();
+    	}
+     
+    	reverse(res.begin(), res.end());
+    	cout<<res.size()<<"\n";
+    	for(auto it: res) cout<<it<<" ";
+     
+    	cout<<"\n";
+     
+     
+     
+    	return 0;
+    }