chore: solved day 15 hard task

hard/day_15/solution.cpp

@@ -1 +1,78 @@
-//write you code here 
+//write you code here 
+#include<bits/stdc++.h>
+using namespace std;
+bool isBip(vector<vector<int> > &adj, int n){
+	vector<int> side(n, -1);
+	bool is_bipartite = true;
+	queue<int> q;
+	for (int st = 0; st < n; ++st) {
+	    if (side[st] == -1) {
+	        q.push(st);
+	        side[st] = 0;
+	        while (!q.empty()) {
+	            int v = q.front();
+	            q.pop();
+	            for (int i = 0; i < adj[v].size() ; i++) {
+	            	int u = adj[v][i];
+	                if (side[u] == -1) {
+	                    side[u] = side[v] ^ 1;
+	                    q.push(u);
+	                } else {
+	                    is_bipartite &= side[u] != side[v];
+	                }
+	            }
+	        }
+	    }
+	}
+	return is_bipartite;
+}
+int main(){
+	int t;
+	cin>>t;
+	while(t--){
+		int n,a,b;
+		cin>>n;
+		vector<pair<int,int> > in(n);
+		bool flag = true;
+		int cnt[n+1] = {0};
+		int i = 0;
+		vector<vector<int> > edge;
+		edge.resize(n+1);
+		while(i<n){
+			cin>>a>>b;
+			in[i].first  = a;
+			in[i].second = b;
+			edge[a].push_back(i);
+			edge[b].push_back(i);
+			if(!flag){
+				i++;
+				continue;
+			}
+			if( a == b)	{
+				flag = false;
+			}
+			cnt[a]++;
+			cnt[b]++;
+			if(cnt[a] > 2 || cnt[b] > 2)	{
+				flag = false;
+			}
+			i++;
+		}
+		if(!flag){
+			cout<<"NO\n";
+			continue;
+		}
+		vector<vector<int> > adj;
+		adj.resize(n);
+		for(i = 1 ; i <= n ; i ++ ){
+			int u = edge[i][0];
+			int v = edge[i][1];
+			adj[u].push_back(v);
+			adj[v].push_back(u);
+		}
+		if(!isBip(adj,n))	cout<<"NO\n";
+		else cout<<"YES\n";
+
+	}
+	return 0;
+}