强联通总结

#include<bits/stdc++.h>
#define REP(i, a, b) for(register int i = (a); i < (b); i++)
#define _for(i, a, b) for(register int i = (a); i <= (b); i++)
using namespace std;

const int MAXN = 2e4 + 10;
const int MAXM = 2e5 + 10;
struct Edge{ int to, next; } e[MAXM];
int head[MAXN], tot, n, m;

int low[MAXN], dfn[MAXN], id;
int belong[MAXN], ins[MAXN], cnt;
int sta[MAXN], top;

void AddEdge(int from, int to)
{
    e[tot] = Edge{to, head[from]};
    head[from] = tot++;
}

void tarjan(int u)
{
    low[u] = dfn[u] = ++id;
    sta[++top] = u; ins[u] = 1;
    for(int i = head[u]; ~i; i = e[i].next)
    {
        int v = e[i].to;
        if(!dfn[v])
        {
            tarjan(v);
            low[u] = min(low[u], low[v]);
        }
        else if(ins[v]) low[u] = min(low[u], low[v]);
    }
    
    if(dfn[u] == low[u])
    {
        ++cnt;
        while(1)
        {
            int v = sta[top--];
            ins[v] = 0;
            belong[v] = cnt;
            if(u == v) break;
        }
    }
}

int main()
{
    memset(head, -1, sizeof(head)); tot = 0;
    scanf("%d%d", &n, &m);
    _for(i, 1, m)
    {
        int u, v;
        scanf("%d%d", &u, &v);
        AddEdge(u, v);
    }
    
    _for(i, 1, n)
        if(!dfn[i])
            tarjan(i);
    printf("%d\n", cnt);
    
    return 0;
}