POJ 1985 Cow Marathon（树的直径&BFS+BFS）

大意：找出图中的相距最远两个点的距离。

思路：由于从任意一个点出发找到最远的一个点，一定在相距最远两点的其中一个端点，所以两次BFS找最远距离。

vector:


#include<map>
#include<queue>
#include<cmath>
#include<cstdio>
#include<stack>
#include<iostream>
#include<cstring>
#include<algorithm>
#define ll long long
#define inf 0x3f3f3f3f
#define eps 1e-8
#include<vector>
#define ls l,mid,rt<<1
#define rs mid+1,r,rt<<1|1
using namespace std;

struct node{
    int to,w;
};
vector<node>q[100010];
char str[2];
int ans,dis[100010];
bool vis[100010];

int bfs(int u){
    memset(dis,0,sizeof(dis));
    memset(vis,false,sizeof(vis));
    queue<int>Q;
    while(!Q.empty())
        Q.pop();
    Q.push(u);
    vis[u] = true;
    int bj = u;
    while(!Q.empty()){
        int t = Q.front();
        Q.pop();
        if(dis[t] > ans){
            ans = dis[t];
            bj = t;
        }
        int len = q[t].size();
        for(int i = 0;i < len; ++ i){
            node tmp = q[t][i];
            if(vis[tmp.to]) continue;
            vis[tmp.to] = true;
            dis[tmp.to ] = dis[t] + tmp.w;
            Q.push(tmp.to);
        }
    }
    return bj;
}

int main(){
    int n,m,i,j,k,a,b,c;
    while(~scanf("%d%d",&n,&m)){

        for(i = 0;i <= n;++ i){
            q[i].clear();
        }
        int star;
        for(i = 0;i < m;++ i){
            scanf("%d%d%d%s",&a,&b,&c,str);
            star = a;
            q[a].push_back({b,c});
            q[b].push_back({a,c});
        }
        ans = 0;
        int cnt = bfs(star);
        ans = 0;
        bfs(cnt);
        printf("%d\n",ans);
    }
    return 0;
}

前向星：


#include<map>
#include<queue>
#include<cmath>
#include<cstdio>
#include<stack>
#include<iostream>
#include<cstring>
#include<algorithm>
#define ll long long
#define inf 0x3f3f3f3f
#define eps 1e-8
#include<vector>
#define ls l,mid,rt<<1
#define rs mid+1,r,rt<<1|1
using namespace std;

struct node{
    int to,w,next;
}q[100010];
char str[2];
int ans,dis[100010],cnt,head[100010];
bool vis[100010];
void bu(int a,int b,int c){
    q[cnt].to = b;
    q[cnt].w = c;
    q[cnt].next=head[a];
    head[a] = cnt++;

    q[cnt].to = a;
    q[cnt].w = c;
    q[cnt].next=head[b];
    head[b] = cnt++;
}
int bfs(int u){
    memset(dis,0,sizeof(dis));
    memset(vis,false,sizeof(vis));
    queue<int>Q;
    while(!Q.empty())
        Q.pop();
    Q.push(u);
    vis[u] = true;
    int bj = u;
    while(!Q.empty()){
        int t = Q.front();
        Q.pop();
        if( dis[t] > ans){
            ans = dis[t];
            bj = t;
        }
        for(int i = head[t] ;~i;i = q[i].next){
            int tmp = q[i].to;
            if(vis[tmp]) continue;
            vis[tmp] = true;
            dis[tmp] = dis[t] + q[i].w;
            Q.push(tmp);
        }
    }
    return bj;
}

int main(){
    int n,m,i,j,k,a,b,c;
    while(~scanf("%d%d",&n,&m)){
        cnt = 0;
        for(i = 1;i <= n;++ i){
            head[i] = -1;
        }
        int star;
        for(i = 0;i < m;++ i){
            scanf("%d%d%d%s",&a,&b,&c,str);
            star = a;
            bu(a,b,c);
        }
        ans = 0;
        int pp = bfs(star);
        ans = 0;
        bfs(pp);
        printf("%d\n",ans);
    }
    return 0;
}