【HAOI 2006】 受欢迎的牛

【题目链接】

          点击打开链接

【算法】

            先用tarjan缩点，然后找出度为零的点，即可

【代码】

             

#include<bits/stdc++.h>
using namespace std;
#define MAXN 500010

struct Edge
{
        int to,nxt;
} e[MAXN];

int i,tot,ans,u,v,timer,top,cnt,n,m;
bool instack[MAXN];
int belong[MAXN],low[MAXN],dfn[MAXN],size[MAXN],
        head[MAXN],stk[MAXN],x[MAXN],y[MAXN],degree[MAXN];

template <typename T> inline void read(T &x)
{
    int f = 1; x = 0;
    char c = getchar();
    for (; !isdigit(c); c = getchar()) { if (c == '-') f = -f; }
    for (; isdigit(c); c = getchar()) x = (x << 3) + (x << 1) + c - '0';
    x *= f;
}
template <typename T> inline void write(T x)
{
    if (x < 0)
    {
        putchar('-');
        x = -x;
    }
    if (x > 9) write(x/10);
    putchar(x%10+'0');
}
template <typename T> inline void writeln(T x)
{
    write(x);
    puts("");
}
inline void add(int u,int v)
{
        tot++;
        e[tot] = (Edge){v,head[u]};
        head[u] = tot;    
}
inline void tarjan(int u)
{
        int i,v;
        dfn[u] = low[u] = ++timer;
        instack[u] = true;
        stk[++top] = u;
        for (i = head[u]; i; i = e[i].nxt)
        {
                v = e[i].to;
                if (!dfn[v])
                {
                        tarjan(v);
                        low[u] = min(low[u],low[v]);
                } else 
                {
                        if (instack[v]) 
                                low[u] = min(low[u],dfn[v]);
                }
        }    
        if (dfn[u] == low[u])
        {
                cnt++;
                while (stk[top+1] != u)
                {
                        instack[stk[top]] = false;
                        belong[stk[top]] = cnt;
                        size[cnt]++;
                        top--;
                }
        }
}

int main() {
        
        read(n); read(m);
        for (i = 1; i <= m; i++)
        {
                read(x[i]); read(y[i]);
                add(x[i],y[i]);    
        }        
        for (i = 1; i <= n; i++)
                if (!dfn[i]) tarjan(i);
        for (i = 1; i <= m; i++)
        {
                if (belong[x[i]] != belong[y[i]])
                        degree[belong[x[i]]]++;        
        }
        for (i = 1; i <= cnt; i++)
        {
                if (!degree[i]) 
                {
                        if (ans) 
                        {
                                ans = 0;
                                break;
                        }
                        ans = size[i];
                }
        }
        
        writeln(ans);
        
        return 0;
    
}