Lưu ý: Các code mẫu dưới đây chỉ mang tính tham khảo và có thể không AC được bài tập này

Code mẫu của flashmt

const fi='';
      fo='';
      maxn=800;
var n,m,re,res,dem,last:longint;
    a:array[1..maxn,1..maxn] of longint;
    d,low,st,r,st1:array[1..maxn] of longint;
    free:array[1..maxn] of boolean;

procedure rf;
var i,j,x,y:longint; 
begin
     assign(input,fi); reset(input);
     readln(n,m);
     for i:=1 to m do
     begin
          readln(x,y); a[x,y]:=1;
     end;
     close(input);
end;

procedure push(i:longint);
begin
     inc(last); st[last]:=i;
end;

function pop:longint;
begin
     pop:=st[last];
     dec(last);
end;

function min(t,u:longint):longint;
begin
     if t<u then min:=t else min:=u;
end;

procedure visit(i:longint);
var k,j,sl:longint; kt:boolean;
begin
     inc(dem); d[i]:=dem; low[i]:=d[i];
     push(i);
     for j:=1 to n do
         if free[j] and (a[i,j]=1) then
            if d[j]>0 then low[i]:=min(low[i],d[j])
            else
            begin
                 visit(j);
                 low[i]:=min(low[i],low[j]);
            end;
     if d[i]=low[i] then
     begin
          inc(re); sl:=0;
          repeat
                j:=pop; inc(sl); st1[sl]:=j;
                free[j]:=false;
                r[j]:=re;
          until j=i;
          for j:=1 to n do
              if r[j]<>re then
              begin
                 kt:=true;
                 for k:=1 to sl do
                     if a[j,st1[k]]=1 then 
                     begin 
                          kt:=false; break;
                     end;
                 if not kt then break;
              end;
          if kt then inc(res);
     end;
end;

procedure pr;
var i:longint;
begin
     last:=0; dem:=0; re:=0;
     fillchar(free,sizeof(free),true);
     for i:=1 to n do
         if d[i]=0 then visit(i);
end;

procedure wf;
var i:longint;
begin
     assign(output,fo); rewrite(output);
     writeln(res);
     close(output);
end;

begin
     rf;
     pr;
     wf;
end.

Code mẫu của happyboy99x

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

#define TR(v,i) for(__typeof((v).begin())i=(v).begin();i!=(v).end();++i)
const int N = 800, INF = 1e9;
vector<int> g[N];
int degIn[N], iscc[N], nscc, n, m;
int timed, low[N], d[N];

void enter() {
    cin >> n >> m;
    for(int i = 0; i < m; ++i) {
        int u, v; cin >> u >> v;
        g[u-1].push_back(v-1);
    }
}

stack<int> st;
void dfs(int u) {
    st.push(u); d[u] = low[u] = timed++;
    TR(g[u], v) if(d[*v] == -1)
        dfs(*v), low[u] = min(low[u], low[*v]);
    else
        low[u] = min(low[u], d[*v]);
    if(low[u] == d[u]) {
        int v;
        do {
            v = st.top(); st.pop();
            iscc[v] = nscc;
        } while(u != v);
        ++nscc;
    }
    d[u] = INF;
}

void buildSuperGraph() {
    for(int u = 0; u < n; ++u) TR(g[u], v)
        if(iscc[u] != iscc[*v]) ++degIn[iscc[*v]];
}

int main() {
    ios::sync_with_stdio(false);
    enter();
    fill(d, d+n, -1);
    for(int u = 0; u < n; ++u) if(d[u] == -1) dfs(u);
    buildSuperGraph();
    cout << count(degIn, degIn+nscc, 0) << '\n';
    return 0;
}

Code mẫu của ladpro98

program messageVOJ;
uses    math;
const   maxn=888;
        maxm=sqr(maxn);
        oo=123456789;
        fi='';

type    e=record
        v,link:longint;
        end;
var     adj,assc,rassc:array[1..maxm] of e;
        head,hssc,rhssc,num,low,stack,ssc,topo:array[1..maxn] of longint;
        avail:array[1..maxn] of boolean;
        n,m,count,ls,cs,dssc,drssc,i,res:longint;

procedure input;
var     inp:text;
        i,x,y:longint;
begin
        assign(inp,fi);
        reset(inp);
        readln(inp,n,m);
        for i:=1 to m do
        begin
                readln(inp,x,y);
                adj[i].v:=y;
                adj[i].link:=head[x];
                head[x]:=i;
        end;
        close(inp);
end;

procedure TarjanDfs(u:longint);
var     i:longint;
        v:e;
begin
        inc(count);num[u]:=count;low[u]:=oo;
        inc(ls);stack[ls]:=u;
        i:=head[u];
        while i<>0 do
        begin
                v:=adj[i];
                if avail[v.v] then
                begin
                        if num[v.v]>0 then
                                low[u]:=min(low[u],num[v.v])
                        else
                        begin
                                TarjanDfs(v.v);
                                low[u]:=min(low[u],low[v.v]);
                        end;
                end;
                i:=v.link;
        end;
        if low[u]>=num[u] then
        begin
                inc(cs);
                repeat
                        i:=stack[ls];dec(ls);
                        ssc[i]:=cs;
                        avail[i]:=false;
                until i=u;
        end;
end;

procedure makeGssc;
var     i,j:longint;
begin
        for i:=1 to n do
        begin
                j:=head[i];
                while j<>0 do
                begin
                        if ssc[adj[j].v]<>ssc[i] then
                        begin
                                inc(dssc);
                                assc[dssc].v:=ssc[adj[j].v];
                                assc[dssc].link:=hssc[ssc[i]];
                                hssc[ssc[i]]:=dssc;
                                inc(drssc);
                                rassc[drssc].v:=ssc[i];
                                rassc[drssc].link:=rhssc[ssc[adj[j].v]];
                                rhssc[ssc[adj[j].v]]:=drssc;
                        end;
                        j:=adj[j].link;
                end;
        end;
end;

procedure topoDfs(v:longint);
var     i:longint;
begin
        avail[v]:=false;
        i:=rhssc[v];
        while i<>0 do
        begin
                if avail[rassc[i].v] then
                        topoDfs(rassc[i].v);
                i:=rassc[i].link;
        end;
        inc(count);
        topo[count]:=v;
end;

procedure dfs(u:longint);
var     i:longint;
begin
        avail[u]:=false;
        i:=hssc[u];
        while i>0 do
        begin
                if avail[assc[i].v] then
                        dfs(assc[i].v);
                i:=assc[i].link;
        end;
end;

begin
        input;
        for i:=1 to n do avail[i]:=true;
        for i:=1 to n do if avail[i] then
                TarjanDfs(i);
        makeGssc;
        for i:=1 to cs do avail[i]:=true;
        count:=0;
        for i:=1 to cs do if avail[i] then
                topoDfs(i);
        for i:=1 to cs do avail[i]:=true;
        for i:=1 to count do if avail[topo[i]] then
        begin
                inc(res);
                dfs(topo[i]);
        end;
        write(res);
end.

Code mẫu của RR

#include <cstdio>
#include <iostream>
#include <algorithm>
#include <vector>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <cstring>
#include <cstdlib>
#include <cmath>
#include <string>
#include <memory.h>
#include <sstream>
#include <complex>
#include <iomanip>

#define REP(i,n) for(int i = 0, _n = (n); i < _n; i++)
#define REPD(i,n) for(int i = (n) - 1; i >= 0; i--)
#define FOR(i,a,b) for (int i = (a), _b = (b); i <= _b; i++)
#define FORD(i,a,b) for (int i = (a), _b = (b); i >= _b; i--)
#define FOREACH(it,c) for (__typeof((c).begin()) it=(c).begin();it!=(c).end();it++)
#define RESET(c,x) memset (c, x, sizeof (c))

#define sqr(x) ((x) * (x))
#define PB push_back
#define MP make_pair
#define F first
#define S second
#define ALL(c) (c).begin(), (c).end()
#define SIZE(c) (c).size()

#define DEBUG(x) { cerr << #x << " = " << x << endl; }
#define PR(a,n) {cerr<<#a<<" = "; FOR(_,1,n) cerr << a[_] << ' '; cerr <<endl;}
#define PR0(a,n) {cerr<<#a<<" = ";REP(_,n) cerr << a[_] << ' '; cerr << endl;}

using namespace std;

const double PI = 2.0 * acos (0.0);

typedef long long LL;
typedef pair <int, int> PII;

template <class T> inline T MAX (T a, T b) { if (a > b) return a; return b; }
template <class T> inline T MIN (T a, T b) { if (a < b) return a; return b; }

// ptrrsn_1's template
const int MAXV = 1011;
int dfs_num[MAXV + 5], dfs_low[MAXV + 5], visited[MAXV + 5];
int dfsNumberCounter, numSCC;
int V;
vector <int> S;
vector <int> G[MAXV + 5];

vector <int> ls[MAXV + 5];
int reg[MAXV + 5];
bool vao[MAXV + 5];

void tarjanSCC(int u) {
    dfs_low[u] = dfs_num[u] = dfsNumberCounter++;
    S.PB(u);
    visited[u] = 1;
    REP (j, SIZE(G[u])) {
        int v = G[u][j];
        if (dfs_num[v] == -1) tarjanSCC(v);
        if (visited[v]) dfs_low[u] = min(dfs_low[u], dfs_low[v]);
    }
    if (dfs_low[u] == dfs_num[u]) {
        ++numSCC;
        while (1) {
            int v = S.back(); S.pop_back(); visited[v] = 0;
            ls[numSCC].PB(v);
            reg[v] = numSCC;
            if (u == v) break;
        }
    }
}

int main() {
    #ifdef LOCAL
        freopen("input.txt", "r", stdin);
    #endif
    scanf("%d", &V);
    int m; scanf("%d", &m);
    while (m--) {
        int u, v; scanf("%d%d", &u, &v);
        --u; --v;
        G[u].PB(v);
    }
    RESET(dfs_num, -1); RESET(dfs_low, 0); RESET(visited, 0);
    dfsNumberCounter = numSCC = 0;
    REP (i, V) if (dfs_num[i] == -1) tarjanSCC(i);

    REP(u,V) 
    REP(i,G[u].size()) {
        int v = G[u][i];
        if (reg[u] != reg[v]) vao[reg[v]] = true;
    }

    int res = 0;
    FOR(i,1,numSCC) if (!vao[i]) ++res;
    printf("%d\n", res);
    return 0;
}

Code mẫu của ll931110

{$MODE DELPHI}
Program MESS;
Const
  input  = '';
  output = '';
  maxn = 800;
Var
  a: array[1..maxn,1..maxn] of boolean;
  free: array[1..maxn] of boolean;
  list,num: array[1..maxn] of integer;
  n,m,count: integer;

Procedure init;
Var
  f: text;
  i,u,v: integer;
Begin
  Assign(f, input);
    Reset(f);

  Readln(f, n, m);
  Fillchar(a, sizeof(a), false);

  For i:= 1 to m do
    Begin
      Readln(f, u, v);
      a[u,v]:= true;
    End;

  Close(f);
End;

Procedure visit(u: integer);
Var
  v: integer;
Begin
  free[u]:= false;
  For v:= 1 to n do
    if a[u,v] and free[v] then
      Begin
        free[v]:= false;
        visit(v);
      End;

  inc(count);
  list[u]:= count;
End;

Procedure solve;
Var
  f: text;
  i,res: integer;
Begin
  count:= 0;
  Fillchar(free, sizeof(free), true);
  For i:= 1 to n do
    if free[i] then visit(i);

  For i:= 1 to n do num[list[i]]:= i;

  res:= 0;
  Fillchar(free, sizeof(free), true);
  For i:= n downto 1 do
    if free[num[i]] then
      Begin
        inc(res);
        visit(num[i]);
      End;

  Assign(f, output);
    Rewrite(f);
    Writeln(f, res);
  Close(f);
End;

Begin
  init;
  solve;
End.

Code mẫu của khuc_tuan

#include "iostream"

using namespace std;

    int n,m;
    bool a[888][888];
    bool mark[888], visited[888];

    void dfs(int i){
        visited[i]=true;
        for(int j=1;j<=n;++j) if(a[i][j] && !visited[j]){
            dfs(j);
        }
    }

    int main() {
        int u,v,dem=0;
        scanf("%d%d",&n,&m);
        for(int i=0;i<m;++i){
            scanf("%d%d",&u,&v);
            a[u][v]=true;
        }
        for(int i=1;i<=n;++i) if(!visited[i]){
            mark[i]=true;
            dfs(i);
        }
        memset( visited, 0, sizeof(visited));
        for(int i=n;i>=1;--i) if(mark[i] && !visited[i]){
            ++dem;
            dfs(i);
        }
        cout << dem << endl;
        return 0;
    }