搜索与图论：树的重心

题目描述

输入样例

9
1 2
1 7
1 4
2 8
2 5
4 3
3 9
4 6

输出样例

4

参考代码

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

using namespace std;

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

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

int ans = N;

void add(int a, int b)
{
    e[idx] = b, ne[idx] = h[a], h[a] = idx++;
}

// 以u为根的子树中点的数量
int dfs(int u)
{
    st[u] = true;   // 标记一下，已经被搜过了
    
    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 >> m;
    memset(h, -1, sizeof h);
    for (int i = 0; i < n; i++)
    {
        int a, b;
        cin >> a >> b;
        add(a, b), add(b, a);
    }
    dfs(1);
    
    cout << ans << endl;
    
    return 0;
}