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

#include <iostream>
#include <algorithm>
#include <cstdio>
#include <map>
using namespace std;
const int base = int(1e9 + 7);

int n, a[2000200], d[2000200], p[2000], P;
map <int,int> factors;

int isPrime(int n)
{
    for (int i = 2; i * i <= n; i++)
        if (n % i == 0) return 0;
    return 1;
}

int main()
{
    for (int i = 2; i * i <= 2000000; i++)
        if (isPrime(i)) p[P++] = i;

    cin >> n;
    for (int i = 1; i <= n; i++) scanf("%d", a + i);

    for (int i = 1; i <= n; i++)
        if (!d[i])
        {
            int x = i, cnt = 0;
            while (!d[x]) d[x] = 1, cnt++, x = a[x];

            for (int j = 0; j < P && p[j] * p[j] <= cnt; j++)
                if (cnt % p[j] == 0)
                {
                    int y = 0;
                    while (cnt % p[j] == 0) cnt /= p[j], y++;
                    factors[p[j]] = max(factors[p[j]], y);
                }

            if (cnt > 1) factors[cnt] = max(factors[cnt], 1);
        }

    long long ans = 1;
    for (map <int,int>::iterator it = factors.begin(); it != factors.end(); it++)
        for (int i = 1; i <= it -> second; i++)
            ans = ans * it -> first % base;

    cout << ans << endl;
}

Code mẫu của ladpro98

program PERC;
uses    math;
const   fi = '';
        maxn = 2000600;
        B = trunc(1e9) + 7;
var     a,S,Mul,prime,P,F,Power: array[1..maxn] of longint;
        done,chk:array[1..maxn] of boolean;
        m,n,d:longint;

procedure Input;
var     inp:text;
        i:longint;
begin
        assign(inp,fi);reset(inp);
        readln(inp,n);
        for i:=1 to n do read(inp,a[i]);
        close(inp);
end;

procedure InitMultipliers;
var     i, top, V, cnt: longint;
begin
        for i:=1 to n do if not done[i] then begin
                done[i] := true; cnt := 0;
                top := 1; S[top] := i; V := i;
                repeat
                        inc(cnt);
                        V := a[V];
                        inc(top); S[top] := V;
                until V = i;
                for V:=1 to top do done[S[V]] := true;
                inc(m); Mul[m] := cnt;
        end;
end;

procedure InitSieve;
var     i,j:longint;
begin
        for i:=2 to trunc(sqrt(N))+1 do
        if prime[i] = 0 then begin
                j := i*i;
                while j<=N do begin
                        prime[j] := i;
                        j := j + i;
                end;
        end;
        for i:=2 to N do
        if prime[i] = 0 then begin
                prime[i] := i;
                inc(d); P[d] := i;
        end;
end;

procedure Process;
var     i, j, top, Divisor: longint;
begin
        for i:=1 to m do begin
                top := 0;
                while Mul[i] > 1 do begin
                        Divisor := prime[Mul[i]];
                        if not chk[Divisor] then begin
                                chk[Divisor] := true;
                                inc(top); S[top] := Divisor;
                        end;
                        inc(F[Divisor]);
                        Mul[i] := Mul[i] div Divisor;
                end;
                for j:=1 to top do begin
                        Power[S[j]] := max(Power[S[j]], F[S[j]]);
                        chk[S[j]] := false;
                        F[S[j]] := 0;
                end;
        end;
end;

function Pow(i: int64; pp: longint): int64;
var     t: int64;
begin
        if pp = 0 then exit(1);
        if pp = 1 then exit(i mod b);
        t := Pow(i, pp shr 1);
        t := t * t mod B;
        if odd(pp) then exit(t * i mod B);
        exit(t);
end;

procedure Output;
var     i:longint;
        res:int64;
begin
        res:=1;
        for i:=1 to d do
        if power[p[i]]>0 then
        res := res*pow(P[i], power[p[i]]) mod B;
        write(res);
end;

begin
        Input;
        InitMultipliers;
        InitSieve;
        Process;
        Output;
end.

Code mẫu của RR

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

#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 REP(i,a) for(int i=0,_a=(a); i<_a; i++)
#define FORN(i,a,b) for(int i=(a),_b=(b);i<_b;i++)
#define DOWN(i,a,b) for(int i=a,_b=(b);i>=_b;i--)
#define SET(a,v) memset(a,v,sizeof(a))
#define sqr(x) ((x)*(x))
#define ll long long
#define F first
#define S second
#define PB push_back
#define MP make_pair

#define DEBUG(x) cout << #x << " = "; cout << x << endl;
#define PR(a,n) cout << #a << " = "; FOR(_,1,n) cout << a[_] << ' '; cout << endl;
#define PR0(a,n) cout << #a << " = "; REP(_,n) cout << a[_] << ' '; cout << endl;
using namespace std;

//Buffer reading
int INP,AM,REACHEOF;
#define BUFSIZE (1<<12)
char BUF[BUFSIZE+1], *inp=BUF;
#define GETCHAR(INP) { \
    if(!*inp) { \
        if (REACHEOF) return 0;\
        memset(BUF,0,sizeof BUF);\
        int inpzzz = fread(BUF,1,BUFSIZE,stdin);\
        if (inpzzz != BUFSIZE) REACHEOF = true;\
        inp=BUF; \
    } \
    INP=*inp++; \
}
#define DIG(a) (((a)>='0')&&((a)<='9'))
#define GN(j) { \
    AM=0;\
    GETCHAR(INP); while(!DIG(INP) && INP!='-') GETCHAR(INP);\
    if (INP=='-') {AM=1;GETCHAR(INP);} \
    j=INP-'0'; GETCHAR(INP); \
    while(DIG(INP)){j=10*j+(INP-'0');GETCHAR(INP);} \
    if (AM) j=-j;\
}
//End of buffer reading

const long double PI = acos((long double) -1.0);
const int MN = 2000111;
const long long MOD = 1000000007;

int a[MN];
int sieve[MN], cnt[MN];
bool visited[MN];

void init() {
    FOR(i,2,2000) if (!sieve[i]) {
        int j = i*i;
        while (j <= 2000000) {
            sieve[j] = i;
            j += i;
        }
    }
}

void update(int n) {
    while (n > 1) {
        int p = sieve[n], now = 0;
        if (p == 0) p = n;
        while (n % p == 0) {
            n /= p;
            ++now;
        }

        cnt[p] = max(cnt[p], now);
    }
}

int main() {
    init();
    int n;
    while (scanf("%d", &n) == 1) {
        FOR(i,1,n) scanf("%d", &a[i]);
        memset(visited, 0, sizeof visited);

        FOR(i,1,n) if (!visited[i]) {
            int len = 1;
            visited[i] = true;
            int u = a[i];

            while (u != i) {
                ++len;
                visited[u] = true;
                u = a[u];
            }

            update(len);
        }

        long long res = 1;
        FOR(i,2,n) if (!sieve[i]) {
            while (cnt[i]) {
                --cnt[i];
                res = (res * i) % MOD;
            }
        }
        cout << res << endl;
    }
    return 0;
}

Code mẫu của hieult

#include <set>
#include <map>
#include <list>
#include <cmath>
#include <queue>
#include <stack>
#include <cstdio>
#include <string>
#include <vector>
#include <cstdlib>
#include <cstring>
#include <sstream>
#include <iomanip>
#include <iostream>
#include <algorithm>

#include <ctime>
#include <deque>
#include <bitset>
#include <cctype>
#include <utility>
#include <cassert>

using namespace std;

typedef long long ll;
typedef double ld;
typedef unsigned int ui;
typedef unsigned long long ull;

#define Rep(i,n) for(int i = 0; i < (n); ++i)
#define Repd(i,n) for(int i = (n)-1; i >= 0; --i)
#define For(i,a,b) for(int i = (a); i <= (b); ++i)
#define Ford(i,a,b) for(int i = (a); i >= (b); --i)
#define Fit(i,v) for(__typeof((v).begin()) i = (v).begin(); i != (v).end(); ++i)
//#define Fitd(i,v) for(__typeof((v).rbegin()) i = (v).rbegin(); i != (v).rend(); ++i)
#define mp make_pair
#define pb push_back
#define fi first
#define se second
#define sz(a) ((int)(a).size())
#define all(a) (a).begin(), (a).end()
#define ms(a,x) memset(a, x, sizeof(a))

template<class F, class T> T convert(F a, int p = -1) { stringstream ss; if (p >= 0) ss << fixed << setprecision(p); ss << a; T r; ss >> r; return r; }
template<class T> T gcd(T a, T b) { T r; while (b != 0) { r = a % b; a = b; b = r; } return a; }
template<class T> T lcm(T a, T b) { return a / gcd(a, b) * b; }
template<class T> T sqr(T x) { return x * x; }
template<class T> T cube(T x) { return x * x * x; }
template<class T> int getbit(T s, int i) { return (s >> i) & 1; }
template<class T> T onbit(T s, int i) { return s | (T(1) << i); }
template<class T> T offbit(T s, int i) { return s & (~(T(1) << i)); }
template<class T> int cntbit(T s) { return s == 0 ? 0 : cntbit(s >> 1) + (s & 1); }

const int bfsz = 1 << 16;
char bf[bfsz + 5];
int rsz = 0;
int ptr = 0;
char gc() {
    if (rsz <= 0) {
        ptr = 0;
        rsz = (int) fread(bf, 1, bfsz, stdin);
        if (rsz <= 0)
            return EOF;
    }
    --rsz;
    return bf[ptr++];
}
void ga(char &c) {
    c = EOF;
    while (!isalpha(c))
        c = gc();
}
int gs(char s[]) {
    int l = 0;
    char c = gc();
    while (isspace(c))
        c = gc();
    while (c != EOF && !isspace(c)) {
        s[l++] = c;
        c = gc();
    }
    s[l] = '\0';
    return l;
}
template<class T> bool gi(T &v) {
    v = 0;
    char c = gc();
    while (c != EOF && c != '-' && !isdigit(c))
        c = gc();
    if (c == EOF)
        return false;
    bool neg = c == '-';
    if (neg)
        c = gc();
    while (isdigit(c)) {
        v = v * 10 + c - '0';
        c = gc();
    }
    if (neg)
        v = -v;
    return true;
}

typedef pair<int, int> II;

const ld PI = acos(-1.0);
const ld eps = 1e-9;

const int dr[] = {0, +1, 0, -1};
const int dc[] = {+1, 0, -1, 0};

const int inf = (int)1e9 + 5;
const ll linf = (ll)1e16 + 5;
const ll mod = (ll)1e9 + 7;

#define maxn 2000005

int a[maxn], num[maxn], flag[maxn];
int n;

ll mul(ll x, int k){
    if(k == 0) return 1;
    if(k & 1) return mul(x, k - 1) * x % mod;
    ll t = mul(x, k >> 1);
    return t * t % mod;
}

void cal(int x){
    for(int i = 2; i * i <= x; i++) if(x % i == 0){
        int run = 0;
        while(x % i == 0){
            run++;
            x /= i;
        }
        num[i] = max(num[i], run);
    }
    if(x > 1) num[x] = max(num[x], 1);
}

int  main()
{
//  freopen("in.txt", "r", stdin);
    gi(n);
    For(i, 1, n) gi(a[i]);
    ms(num, 0); ms(flag, 0);
    For(i, 1, n) if(!flag[i]){
        int run = 1, u = a[i];
        flag[i] = 1;
        while(u != i){
            flag[u] = 1;
            run++;
            u = a[u];
        }
        cal(run);
    }

    ll res = 1;
    For(i, 2, maxn - 1){
        res = (res * mul(i, num[i])) % mod;
    }
    cout << res;

    return 0;
}