BZOJ1051 [HAOI2006]受欢迎的牛

看看就发现了是道一眼题、、然后蒟蒻写了1h、、

先tarjan，然后缩点重新建图，最后扫一遍就可以了、、

（为什么加inline比不加要慢、、为什么调用全局变量比开局部变量要慢、、都是开O2的原因嘛！！！>_<）

/**************************************************************
    Problem: 1051
    User: rausen
    Language: C++
    Result: Accepted
    Time:48 ms
    Memory:2132 kb
****************************************************************/
 
#include <cstdio>
#include <algorithm>
 
using namespace std;
const int N = 10005;
const int M = 50005;
struct edges{
    int next, to;
}e[M], E[M];
int n, m;
int first[N], FIRST[N], tot, TOT;
int low[N], dfn[N], s[N], w[N], cnt, top;
int Cnt[N], num;
bool vis[N], inq[N];
 
inline int read(){
    int x = 0;
    char ch = getchar();
    while (ch < '0' || ch > '9')
        ch = getchar();
         
    while (ch >= '0' && ch <= '9'){
        x = x * 10 + ch - '0';
        ch = getchar();
    }
    return x;
}
 
inline void add_edge(int x, int y){
    e[++tot].next = first[x];
    first[x] = tot;
    e[tot].to = y;
}
 
inline void ADD_EDGE(int x, int y){
    E[++TOT].next = FIRST[x];
    FIRST[x] = TOT;
    E[TOT].to = y;
}
 
void DFS(int p){
    low[p] = dfn[p] = ++cnt;
    vis[p] = inq[p] = 1;
    s[++top] = p;
    int x, y;
    for (x = first[p]; x; x = e[x].next)
        if (!vis[(y = e[x].to)]){
            DFS(y);
            low[p] = min(low[p], low[y]);
        }else
        if (inq[y]) low[p] = min(low[p], dfn[y]);
    if (low[p] == dfn[p]){
        ++num, y = 0;
        while (y != p){
            inq[    (y = s[top--])] = 0;
            w[y] = num;
            ++Cnt[num];
        }
    }
}
 
void rebuild_graph(){
    int x, y; 
    for (int i = 1; i <= n; ++i)
        for (x = first[i]; x; x = e[x].next)
            if (w[i] != w[(y = e[x].to)])
                ADD_EDGE(w[i], w[y]);
}
 
void tarjan(){
    for (int i = 1; i <= n; ++i)
        if (!vis[i]) DFS(i);
    rebuild_graph();
}
 
int work(){
    int res = 0;
    for (int i = 1; i <= num; ++i)
        if (!FIRST[i]){
            if (res) return 0;
            else res = Cnt[i];
        }
    return res;
}
 
int main(){
    n = read(), m = read();
    int X, Y;
    for (int i = 1; i <= m; ++i){
        X = read(), Y = read();
        add_edge(X, Y);
    }
    tarjan();
    printf("%d
", work());
    return 0;
}