树的重心

#include<cstring>
#include<iostream>
#include<algorithm>

using namespace std;

const int N = 100010, M = N * 2;

int n, m;
bool st[N];
int ans = N;
int h[N], e[M], ne[M], idx;

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, res = 0;
    for(int i = h[u]; i != -1; 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(ans, res);
    return sum;
}

int main() {
    cin >> n;
    memset(h, -1, sizeof h);

    for(int i = 0; i < n - 1; i ++) {
        int a, b;
        cin >> a >> b;
        add(a, b), add(b, a);
    }

    dfs(1);
    cout << ans << endl;
    return 0;
}