学步园

自己暴了一下不过     转一个 bfs...   

#include <cstdio>
#include <vector>
#include <queue>
#include <cstring>
#define maxn 10010
using namespace std;

struct node
{
    int v,dist;
    node() {}
    node(int _v, int _dist)
    {
        v = _v;
        dist = _dist;
    }
} aux;
bool visited[maxn];
vector<int> L[maxn];

node search(int v)
{
    node ans = node(v,0);
    queue<node> Q;
    Q.push(node(v,0));
    memset(visited,false,sizeof(visited));
    visited[v] = true;
    while(!Q.empty())
    {
        aux = Q.front();
        Q.pop();
        ans = aux;
        for(int i = L[aux.v].size()-1; i >= 0; --i)
        {
            if(visited[L[aux.v][i]])
                continue;
            visited[L[aux.v][i]] = true;
            Q.push(node(L[aux.v][i], aux.dist+1));
        }
    }
    return ans;
}

int main()
{
    int N,u,v;
    scanf("%d",&N);
    for(int i = 1; i < N; ++i)
    {
        scanf("%d%d",&u,&v);
        L[u].push_back(v), L[v].push_back(u);
    }
    printf("%d\n",search(search(1).v).dist);
    return 0;
}