POJ 2631 Roads in the North 解题报告 （树的直径）

传送门：http://poj.org/problem?id=2631

这题是求树的直径的裸题，没啥的，原理也挺简单，两次BFS就出了，需要注意的可能就是数据范围，数据读入吧，没判断重边也没啥坑好像……

上一波C艹实现

#include 
#include 
#include 
#include 
using namespace std;
const int MAXN = 20010;

struct node{
    int v, w, next;
}edge[MAXN];
int head[MAXN], cnt = 0, dis[MAXN];

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

int BFS(int x){
    memset(dis, 0x3f, sizeof(dis));
    queue q;
    dis[x] = 0;
    q.push(x);
    while(!q.empty()){
        int temp = q.front();
        q.pop();
        for(int i = head[temp]; i != 0;i = edge[i].next){
            if(dis[edge[i].v] > dis[temp] + edge[i].w){
                dis[edge[i].v] = dis[temp] + edge[i].w;
                q.push(edge[i].v);
                if(dis[edge[i].v] > dis[x])
                    x = edge[i].v;
            }
        }
    }
    return x;
}
int main()
{
    //freopen("input.txt", "r", stdin);
    int u, v, w;
    memset(head, 0, sizeof(head));
    while(scanf("%d %d %d", &u, &v, &w) != EOF){
        add(u, v, w);
        add(v, u, w);
    }
    int z = BFS(1);
    int e = BFS(z);
    printf("%d\n", dis[e]);
    return 0;
}