1985（POJ）

树的直径（树中最左边端点到最右边端点的距离）

#include<iostream>

#include<cstdio>

#include<string.h>

#include<string>

#include<stack>

#include<set>

#include<algorithm>

#include<cmath>

#include<vector>

#include<map>

#include<sstream>

#include<queue>

#define ll __int64

#define lll unsigned long long

#define MAX 1000009

#define MAXN 2009

#define eps 1e-8

#define INF 0x7fffffff

#define mod 1000000007

#define clr(a) memset(a,0,sizeof(a))

#define clr1(a) memset(a,-1,sizeof(a))

#define lson l , m , rt << 1

#define rson m + 1 , r , rt << 1 | 1

using namespace std;

/*

想法： 树的直径（树中最左边端点到最右边端点的距离）

题意：求树中最长的路径

*/

struct node

{

int v;

int w;

int next;

int len;

}edge[MAX];

queue<int>Q;

int dist[MAX];

int head[MAX];

int vis[MAX];

int tot,top;

int n,m;

int point;

void init()

{

clr(vis);

clr1(head);

tot = 0;

top = 0;

}

void add_edge(int u,int v,int w)

{

edge[tot].v = v;

edge[tot].w = w;

edge[tot].len = 0;

edge[tot].next = head[u];

head[u] = tot++;

}

int bfs(int start)

{

//cout<<start<<endl;

while(!Q.empty())Q.pop();

clr(dist);

vis[start] = 1;

int ans = -INF;

Q.push(start);

while(!Q.empty())

{

int u = Q.front();Q.pop();

//cout<<u<<endl;

for(int i = head[u];i!=-1;i = edge[i].next)

{

int v = edge[i].v;

//cout<<u<<" "<<v<<endl;

if(!vis[v])

{

vis[v] = 1;

dist[v]= dist[u] + edge[i].w;

if(dist[v]>ans)

{

ans = dist[v];

point = v;

}

Q.push(v);

}

}

}

return ans;

}

int main()

{

#ifdef ONLINE_JUDGE

#else

freopen("an.txt","r", stdin);

#endif

int u,v,w;

while(~scanf("%d%d",&n,&m))

{

init();

for(int i = 0;i<m;i++)

{

char s;

scanf("%d%d%d%c",&u,&v,&w,&s);

getchar();

add_edge(u,v,w);

add_edge(v,u,w);

}

bfs(1);

clr(vis);

//cout<<point<<endl;

printf("%d\n",bfs(point));

}

return 0;

}