POJ 1655 Balancing Act (树形dp 树的重心)

#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;
int const MAX = 20005;
int const INF = 0x3fffffff;

struct EDGE
{
    int to, next;
}e[MAX * 2];

int head[MAX], dp[MAX];
bool vis[MAX];
int n, cnt, num, ans, b;

void Add(int u, int v)
{
    e[cnt].to = v;
    e[cnt].next = head[u];
    head[u] = cnt ++;
}

void Init()
{
    cnt = 0;
    n = 0;
    num = INF;
    ans = n;
    memset(head, -1, sizeof(head));
    memset(dp, 0, sizeof(dp));
    memset(vis, false, sizeof(vis));
}

void DFS(int rt)
{
    vis[rt] = true;
    dp[rt] = 0;
    int b = 0;
    for(int i = head[rt]; i != -1; i = e[i].next)
    {
        int son = e[i].to;
        if(!vis[son])
        {
            DFS(son);
            dp[rt] += dp[son] + 1;
            b = max(b, dp[son] + 1); 
        }
    }
    b = max(b, n - dp[rt] - 1);
    if(b < num || (b == num && rt < ans))
    {
        num = b;
        ans = rt;
    }
    return; 
}

int main()
{
    int T;
    scanf("%d", &T);
    while(T--)
    {
        Init();
        int st;
        scanf("%d", &n);
        for(int i = 0; i < n - 1; i++)
        {
            int u, v;
            scanf("%d %d", &u, &v);
            Add(v, u);   
            Add(u, v);
            st = u;
        }
        DFS(st);
        printf("%d %d\n", ans, num);
    }
}