POJ1985：树的直径

POJ1985

题解：

• 求树中最长路。从任意点出发BFS找最远的点，再从最远的那个点BFS找最远的点便是直径。

代码：

#include <cstring>
#include <algorithm>
#include <iostream>
#include <cstdio>
#include <queue>
#include <vector>
using namespace std;
int const N = 50000 + 10;
int n,m,last,num;
int dis[N];
struct Edge
{
    int u,dist; 
    Edge(int a,int b):u(a),dist(b){}
};
vector<Edge>G[N];
int BFS(int u){
    queue<Edge>q;
    q.push(Edge(u,0));
    memset(dis,-1,sizeof(dis));
    dis[u] = last = 0;
    num = 1;
    while(!q.empty()){
        Edge p = q.front(); q.pop();
        int x = p.u;
        for(int i=0;i<G[x].size();i++){
            Edge e = G[x][i];
            int y = e.u;
            if(dis[y] == -1){
                dis[y] = max(dis[y],dis[x] + e.dist);
                if(dis[y] > last)
                    last = dis[y],  num = y;
                q.push(Edge(y,e.dist));
            }
        }
    }
    return last;
}
int main(){
    scanf("%d%d",&n,&m);
    for(int i=0;i<m;i++){
        int u,v,d;
        char t;
        scanf("%d%d%d %c",&u,&v,&d,&t);
        G[u].push_back(Edge(v,d));
        G[v].push_back(Edge(u,d));
    }
    BFS(1);
    printf("%d\n",BFS(num));
}