poj 3177 Redundant Paths(Tarjan，边双连通分量)

#include <stdio.h>
#include <string.h>
#include <algorithm>
using namespace std;

const int MAXN = 5010;
const int MAXM = 20010;
struct Edge
{
int to,next;
bool cut;
} edge[MAXM];
int head[MAXN],tot;
int Low[MAXN],Dfn[MAXN],Stack[MAXN],Belong[MAXN];
int Index,top;
int block;
int bridge;

void addedge(int u, int v)
{
edge[tot].to = v;
edge[tot].next = head[u];
edge[tot].cut = false;
head[u] = tot++;
}

void init()
{
memset(head,-1,sizeof(head));
tot = 0;
}

void Tarjan(int u, int pre)
{
int v;
Low[u] = Dfn[u] = ++Index;
Stack[top++] = u;
for(int i = head[u]; i != -1; i = edge[i].next)
{
v = edge[i].to;
if(v == pre) continue;
if(!Dfn[v])
{
Tarjan(v,u);
if(Low[u] > Low[v]) Low[u] = Low[v];
if(Low[v] > Dfn[u])
{
bridge++;
edge[i].cut = true;
edge[i^1].cut = true;
}
}
else if(Low[u] > Dfn[v])
Low[u] = Dfn[v];
}
if(Low[u] == Dfn[u])
{
block++;
do
{
v = Stack[--top];
Belong[v] = block;
}while(v != u);
}
}

int du[MAXN];
void solve(int n)
{
memset(Dfn,0,sizeof(Dfn));
Index = top = block = 0;
Tarjan(1,0);
int ans = 0;
memset(du,0,sizeof(du));
for(int i = 1; i <= n; ++i)
{
for(int j = head[i]; j != -1; j = edge[j].next)
{
if(edge[j].cut)
du[Belong[i]]++;
}
}
for(int i = 1; i <= block; ++i)
if(du[i] == 1)
++ans;
printf("%d\n",(ans+1)/2);
}

int main()
{
int n,m,u,v;
while(scanf("%d %d",&n,&m) != EOF)
{
init();
while(m--)
{
scanf("%d %d",&u,&v);
addedge(u,v);
addedge(v,u);
}
solve(n);
}
return 0;
}