POJ3895 Cycles of Lanes，DFS

/*******************************************************************************
# Author : Neo Fung
# Email : neosfung@gmail.com
# Last modified: 2012-06-13 16:07
# Filename: acm.cpp
# Description :
******************************************************************************/
#ifdef _MSC_VER
#define DEBUG
#define _CRT_SECURE_NO_DEPRECATE
#endif

#include <fstream>
#include <stdio.h>
#include <iostream>
#include <string.h>
#include <string>
#include <limits.h>
#include <algorithm>
#include <math.h>
#include <numeric>
#include <functional>
#include <ctype.h>
#include <vector>
using namespace std;

const int kMAX=10010;
const double kEPS=10E-6;
vector<int> vec[kMAX];
bool visit[kMAX];
int ans,step[kMAX];

void dfs(const int &x,const int &t)
{
visit[x]=true;
step[x]=t;
for(size_t i=0;i<vec[x].size();++i)
{
if(!visit[vec[x][i]])
dfs(vec[x][i],t+1);
else
ans=max(ans,step[vec[x][i]]-step[x]+1);
}
}

int main(void)
{
#ifdef DEBUG
freopen("../stdin.txt","r",stdin);
freopen("../stdout.txt","w",stdout);
#endif

int n,m,ncase=1,u,v;
scanf("%d",&ncase);

while(~scanf("%d%d",&n,&m) && n)
{
for(int i=1;i<=n;++i)
vec[i].clear();
memset(visit,false,sizeof(visit));
memset(step,0,sizeof(step));
ans=0;

while(m--)
{
scanf("%d%d",&u,&v);
vec[u].push_back(v);
vec[v].push_back(u);
}

for(int i=1;i<=n;++i)
if(!visit[i])
dfs(i,0);
printf("%d\n",ans);

}

return 0;
}