题目链接

题意：





题解：









AC代码

/*
    Author:zzugzx
    Lang:C++
    Blog:blog.csdn.net/qq_43756519
*/
#include<bits/stdc++.h>
using namespace std;
#define fi first
#define se second
#define pb push_back
#define mp make_pair
#define all(x) (x).begin(),(x).end()
#define endl '\n'
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
const int mod=1e9+7;
//const int mod=998244353;
const double eps = 1e-10;
const double pi=acos(-1.0);
const int maxn=1e6+10;
const ll inf=0x3f3f3f3f;
const int dir[4][2]={{0,1},{1,0},{0,-1},{-1,0}};

int dp[maxn][2];
vector<int> g[maxn];
void dfs(int u,int fa){
    dp[u][1]=1;
    for(auto v:g[u]){
        if(v==fa)continue;
        dfs(v,u);
        dp[u][1]+=dp[v][0];
        dp[u][0]+=max(dp[v][1],dp[v][0]);
    }
}
int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);cout.tie(0);
    //freopen("in.txt","r",stdin);
    //freopen("out.txt","w",stdout);
    int n,s;
    cin>>n>>s;
    for(int i=1,u,v;i<n;i++){
        cin>>u>>v;
        g[u].pb(v);
        g[v].pb(u);
    }
    dfs(s,0);
    cout<<dp[s][1]<<endl;
    return 0;
}