CF1174F Ehab and the Big Finale

https://codeforces.com/contest/1174/problem/F

#include<bits/stdc++.h> using namespace std; const int maxn=2e5+5; vector<int> v[maxn],h; int sz[maxn],dep[maxn],depx; //sz数组存储以i为根节点的树拥有多少结点(包括i本身) int pre(int node,int p){ sz[node]=1; for(int u:v[node]){ if(u!=p){ dep[u]=dep[node]+1; sz[node]+=pre(u,node); } } return sz[node]; } int query(char c,int u){ printf("%c %d\n",c,u); fflush(stdout); int ans; scanf("%d",&ans); return ans; } void get(int node){ h.push_back(node); int big=-1; for(int u:v[node]){ if(sz[node]>sz[u]&&(big==-1||(sz[u]>sz[big]))) big=u; } if(big!=-1) get(big); } int dfs(int node){ h.clear(); get(node); int depy=(depx+dep[h.back()]-query('d',h.back()))/2,y=h[depy-dep[node]]; if (depx==depy) return y; return dfs(query('s',y)); } int main(){ //freopen("datain.txt", "r", stdin); int n;scanf("%d",&n); for(int i=1;i<n;i++){ int a,b; scanf("%d%d",&a,&b); v[a].push_back(b); v[b].push_back(a); } depx=query('d',1); pre(1,0); printf("! %d\n",dfs(1)); fflush(stdout); }