POJ Cow Marathon(树的直径)

/*
问题：求树的直径
方法：树形DP
*/
#include<cstdio>
#include<algorithm>
using namespace std;
const int N = 5e4 + 5;
struct edge {
    int to, nex, wi;
} es[N << 1];
bool vis[N];
int head[N], dis[N], c, diameter;
inline void add(int x, int y, int w) {
    c += 1;
    es[c].to = y;
    es[c].nex = head[x];
    es[c].wi = w;
    head[x] = c;
}
void init(int n) {
    c = diameter = 0;
    for(int i = 0; i <= n; i += 1) {
        head[i] = -1;
        vis[i] = false;
        dis[i] = 0;
    }
}
void dp(int u) {
    vis[u] = true;
    for(int i = head[u]; i != -1; i = es[i].nex) {
        int to = es[i].to;
        if(vis[to]) continue;
        dp(to);
        diameter = max(diameter, dis[u] + dis[to] + es[i].wi);
        dis[u] = max(dis[u], dis[to] + es[i].wi);
    }
}
int main() {
    int v, e, x, y, wi;
    char c;
    while(~scanf("%d%d", &v, &e)) {
        init(v);
        for(int i = 0; i < e; i += 1) {
            scanf("%d %d %d %c", &x, &y, &wi, &c);
            add(x, y, wi);
            add(y, x, wi);
        }
        dp(1);
        printf("%d\n", diameter);
    }
    return 0;
}