树的直径：从随意一点出发，BFS找到最远的距离，然后在从该点出发BFS找到最远的距离

#include 
      
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        #include 
        
         #include 
         
          #include 
          
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            #include 
            
             #include 
             
              using namespace std;const int maxn = 10008;const int inf = 0x3ffffff;struct Node { int w, next,to; Node (int x = 0, int y = 0,int u = 0 ){ to = x;w = y;next = u; }}e[maxn*4];int d[maxn],head[maxn],tot;void add(int u,int v,int w){ e[tot] = Node(v,w,head[u]); head[u] = tot++; e[tot] = Node(u,w,head[v]); head[v] = tot++;}int BFS(int u) { memset(d,-1,sizeof(d)); d[u] = 0; int max_int = 0,id = u; queue 
              
                q; q.push(u); while(!q.empty()){ u = q.front();q.pop(); if(d[u] > max_int) { max_int = d[u]; id = u; } for(int i=head[u];i!=-1;i=e[i].next){ if(d[e[i].to] == -1){ d[e[i].to] = d[u] + e[i].w; q.push(e[i].to); } } } // cout << id <