hdu 3534 树形dp

题意：给定一棵树，求出树的直径以及个数。

思路：dfs到结点u时，顺便统计出以u为lca的路径的最长距离和个数。故需要求出每个结点到叶子结点的最长距离以及个数。题目居然不给数据范围，不过实测n最大值为10000。

#include <iostream>
#include <cstring>
#include <cstdio>
using namespace std;

const int N = 10001;
int head[N];
int len[N];
int num[N];
int n, e, ans, cnt;

void init()
{
    e = 0;
    ans = 0;
    cnt = 1;
    memset( head, -1, sizeof(head) );
}

struct Edge 
{
    int v, next, w;
} edge[N << 1];

void addEdge( int u, int v, int w )
{
    edge[e].v = v;
    edge[e].w = w;
    edge[e].next = head[u];
    head[u] = e++;
}

void dfs( int u, int fa )
{
    len[u] = 0;
    num[u] = 1;
    for ( int i = head[u]; i != -1; i = edge[i].next )
    {
        int v = edge[i].v, w = edge[i].w;
        if ( v == fa ) continue;
        dfs( v, u );
        if ( len[u] + len[v] + w > ans )
        {
            ans = len[u] + len[v] + w;
            cnt = num[u] * num[v];
        }
        else if ( len[u] + len[v] + w == ans )
        {
            cnt += num[u] * num[v];
        }
        if ( len[v] + w > len[u] )
        {
            len[u] = len[v] + w;
            num[u] = num[v];
        }      
        else if ( len[v] + w == len[u] )
        {
            num[u] += num[v];
        }
    }
}

int main ()
{
    while ( scanf("%d", &n) != EOF )
    {
        init();
        for ( int i = 1; i < n; i++ )
        {
            int u, v, w;
            scanf("%d%d%d", &u, &v, &w);
            addEdge( u, v, w );
            addEdge( v, u, w );
        }
        dfs( 1, -1 );
        printf("%d %d
", ans, cnt);
    }
    return 0;
}