华华和月月逛公园（牛客小白赛）

https://ac.nowcoder.com/acm/contest/392/I

#include <bits/stdc++.h>
using namespace std;
#define re register
#define ll long long
const int maxn=1e5+10;
void read(int &a)
{
    a=0;
    int d=1;
    char ch;
    while(ch=getchar(),ch>'9'||ch<'0')
        if(ch=='-')
            d=-1;
    a=ch-'0';
    while(ch=getchar(),ch>='0'&&ch<='9')
        a=a*10+ch-'0';
    a*=d;
}
void write(int x)
{
    if(x<0)
        putchar(45),x=-x;
    if(x>9)
        write(x/10);
    putchar(x%10+'0');
}
int head[maxn],low[maxn],dfn[maxn],num,cnt,ans;
struct note
{
    int v,next;
}q[6*maxn];
void add(int u,int v)
{
    q[++num].next=head[u];
    q[num].v=v;
    head[u]=num;
}
void tarjan(int x,int fa)
{
    low[x]=dfn[x]=++cnt;
    for(re int i=head[x];i;i=q[i].next)
    {
        int v=q[i].v;
        if(v==fa)
            continue;
        if(!dfn[v])
        {
            tarjan(v,x);
            low[x]=min(low[v],low[x]);
            if(low[v]>dfn[x])
                ans++;
        }
        else low[x]=min(low[x],dfn[v]);
    }
}
int main()
{
    int n,m;
    read(n);
    read(m);
    for(re int i=0;i<m;i++)
    {
        int a,b;
        read(a);
        read(b);
        add(a,b);
        add(b,a);
    }
    for(re int i=1;i<=n;i++)
    {
        if(!dfn[i])
            tarjan(i,0);
    }
    write(m-ans);
    return 0;
}