POJ1985 树的直径(BFS

 1 #include"bits/stdc++.h"
 2 
 3 #define db double
 4 #define ll long long
 5 #define vl vector<ll>
 6 #define ci(x) scanf("%d",&x)
 7 #define cd(x) scanf("%lf",&x)
 8 #define cl(x) scanf("%lld",&x)
 9 #define pi(x) printf("%d\n",x)
10 #define pd(x) printf("%f\n",x)
11 #define pl(x) printf("%lld\n",x)
12 #define rep(i, a, n) for (int i=a;i<n;i++)
13 #define per(i, a, n) for (int i=n-1;i>=a;i--)
14 #define fi first
15 #define se second
16 using namespace std;
17 typedef pair<int, int> pii;
18 const int N = 1e6 + 5;
19 const int mod = 1e9 + 7;
20 const int MOD = 998244353;
21 const db PI   = acos(-1.0);
22 const db eps  = 1e-10;
23 const int inf = 0x3f3f3f3f;
24 const ll INF  = 0x3fffffffffffffff;
25 int t, n, m;
26 bool vis[N];
27 char ss[200];
28 vector<pii> e[N];
29 queue<int> q;
30 void add(int u,int v,int w){
31     e[u].push_back(pii(v,w));
32 }
33 int ma;
34 int f[N];
35 int s;
36 void BFS(int x)//BFS求最长路
37 {
38     memset(f,0, sizeof(f));
39     memset(vis,0, sizeof(vis));
40     while(!q.empty()) q.pop();
41     ma=0;
42     q.push(x);
43     vis[x]=1;
44     s=x;
45     while(!q.empty()){
46         int u=q.front();q.pop();
47         for(int i=0;i<e[u].size();i++){
48             int v=e[u][i].fi;
49             int w=e[u][i].se;
50             if(!vis[v]){
51                 if(f[v]<f[u]+w) f[v]=f[u]+w;
52                 vis[v]=1;
53                 q.push(v);
54             }
55             if(ma<f[v]) ma=f[v],s=v;
56         }
57     }
58 }
59 int main() {
60 
61     while (scanf("%d%d", &n, &m) == 2) {
62         for(int i=0;i<=n;i++) e[i].clear();
63         for (int i = 0; i < m; i++) {
64             int x, y, z;
65             scanf("%d %d %d %s", &x, &y, &z, ss);
66             add(x,y,z),add(y,x,z);
67         }
68         BFS(1);
69         BFS(s);
70         pi(ma);
71     }
72     return 0;
73 }