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图论-树的直径-DFS&BFS.cpp
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图论-树的直径-DFS&BFS.cpp
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#include<iostream>
#include<cstdio>
#include<algorithm>
#include<vector>
#include<set>
#include<stack>
#include<queue>
#include<map>
#include<cstring>
#include<string>
#include<cmath>
using namespace std;
typedef long long LL;
#define INF 0x3f3f3f3f
#define PI acos(-1.0)
#define pii pair<int,int>
#define all(x) x.begin(),x.end()
#define mem(a,b) memset(a,b,sizeof(a))
#define per(i,a,b) for(int i = a;i <= b;++i)
#define rep(i,a,b) for(int i = a;i >= b;--i)
const int maxn = 3e5;
int n = 0,m = 0;
int ans = 0,maxid = 0;
int dis[maxn+10];
bool vis[maxn+10];
struct node{
int to,val;
node(int x,int y){
to = x;val = y;
}
};
vector<node> vt[maxn+10];
/*
两次DFS,第一次找到离你选的起点(起点随意选择)最远的点(树的直径上的一点,
然后从最远的点DFS,找到最远的距离
或者BFS都可以
*/
void dfs(int s){
vis[s] = true;
int size = vt[s].size();
for(int i = 0;i < size;++i){
if(vis[vt[s][i].to] == false){
dis[vt[s][i].to] = dis[s] + vt[s][i].val;
if(dis[vt[s][i].to] > ans){
maxid = vt[s][i].to;
ans = dis[vt[s][i].to];
}
dfs(vt[s][i].to);
}
}
}
void bfs(int s){
vis[s] = true;
queue<int> que;
que.push(s);
while(!que.empty()){
int u = que.front();
que.pop();
int size = vt[u].size();
for(int i = 0;i < size;++i){
if(vis[vt[u][i].to] == false){
dis[vt[u][i].to] = dis[u] + vt[u][i].val;
vis[vt[u][i].to] = true;
if(dis[vt[u][i].to] > ans){
maxid = vt[u][i].to;
ans = dis[vt[u][i].to];
}
que.push(vt[u][i].to);
}
}
}
}
int main(){
while(~scanf("%d %d",&n,&m)){
memset(dis,0,sizeof(dis));memset(vis,false,sizeof(vis));
per(i,1,m){
int x = 0,y = 0,d = 0;
char ch;
scanf("%d %d %d %c",&x,&y,&d,&ch);
vt[x].push_back(node(y,d));vt[y].push_back(node(x,d));
}
bfs(1);
memset(dis,0,sizeof(dis));memset(vis,false,sizeof(vis));
//cout << "nima " << maxid << endl;
bfs(maxid);
printf("%d\n",ans);
}
return 0;
}