poj2186--tarjan+缩点

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;
typedef long long ll;
#define cls(s,h) memset(s,h,sizeof s)
const int maxn = 1e5 + 7;
int n , m ;
int tot;
struct  edge
{
    int to,from,nxt;
}e[maxn << 1];

int head[maxn];
void add_edge(int u , int v ){
    e[tot].from = u ;
    e[tot].to = v;
    e[tot].nxt = head[u];
    head[u] = tot++;
}

int stack[maxn << 1],low[maxn << 1],dfn[maxn << 1];
int scc,sz[maxn << 1],c[maxn << 1],st,num;
void init(){
    cls(head,-1);
    cls(e,0);
    cls(sz,0);
    scc = 0;
    st = 0;
}


void tanjan(int u){
    stack[++st] = u;
    dfn[u] = low[u] = ++ num;
    for(int i = head[u],v; ~i ;i = e[i].nxt){
        if(c[v = e[i].to]) continue;
        if(dfn[v]) low[u] = min(low[u],dfn[v]);
        else tanjan(v),low[u] = min(low[u],low[v]);
    }
    if(low[u] == dfn[u]){
        c[u] = ++ scc; sz[scc] = 1;
        while(st && u != stack[st])
            //SCC number and the scc ge SCC
            c[stack[st--]] = scc,sz[scc] ++;
        st--;
    }
}

int out[maxn << 1];
int main(int argc, char const *argv[])
{
    init();
    scanf("%d %d",&n,&m);
    for(int i = 1,u,v;i <= m ;i ++)
    scanf("%d %d",&u,&v),add_edge(u,v);
    
    for(int i = 1;i <= n;i ++)
        if(!dfn[i]) tanjan(i);
    for(int i = 0;i < tot;i ++)
        if(c[e[i].from] != c[e[i].to]) out[c[e[i].from]] ++;
    int ans = 0;
    for(int i = 1;i <= scc;i ++)
        if(!out[i]) ans = ans ? -1: i;
    if(~ans) ans = sz[ans];
    else ans = 0;
    printf("%d\n", ans);
    return 0;
}