树的重心

代码

#include <bits/stdc++.h>
using namespace std;
const int N = 100005,M = 200005;
int h[N],e[M],ne[M],idx,n,m,ans = N;
bool st[N];
void add(int a,int b){
    e[idx] = b,ne[idx] = h[a],h[a] = idx ++;
}
int dfs(int u){
    st[u] = true;
    int sum = 1;
    int res = 0;
    for(int i = h[u]; ~i; i = ne[i]){
        int j = e[i];
        if(!st[j]){
            int s = dfs(j);
            res = max(res, s);
            sum += s;
        }
    }
    res = max(res,n - sum);
    ans = min(res,ans);
    return sum;
}
int main(){
    memset(h, -1, sizeof h);
    cin >> n;
    m = n - 1;
    while(m --){
        int a,b;
        cin >> a >> b;
        add(a,b);
        add(b,a);
    }
    dfs(1);
    cout << ans << endl;
    return 0;
}