POJ2369

Permutations
Time Limit: 1000MS   Memory Limit: 65536K
Total Submissions: 3245   Accepted: 1766

Description

We remind that the permutation of some final set is a one-to-one mapping of the set onto itself. Less formally, that is a way to reorder elements of the set. For example, one can define a permutation of the set {1,2,3,4,5} as follows: 
 
This record defines a permutation P as follows: P(1) = 4, P(2) = 1, P(3) = 5, etc. 
What is the value of the expression P(P(1))? It’s clear, that P(P(1)) = P(4) = 2. And P(P(3)) = P(5) = 3. One can easily see that if P(n) is a permutation then P(P(n)) is a permutation as well. In our example (believe us) 
 
It is natural to denote this permutation by P2(n) = P(P(n)). In a general form the defenition is as follows: P(n) = P1(n), Pk(n) = P(Pk-1(n)). Among the permutations there is a very important one — that moves nothing: 
 
It is clear that for every k the following relation is satisfied: (EN)k = EN. The following less trivial statement is correct (we won't prove it here, you may prove it yourself incidentally): Let P(n) be some permutation of an N elements set. Then there exists a natural number k, that Pk = EN. The least natural k such that Pk = EN is called an order of the permutation P. 
The problem that your program should solve is formulated now in a very simple manner: "Given a permutation find its order."

Input

In the first line of the standard input an only natural number N (1 <= N <= 1000) is contained, that is a number of elements in the set that is rearranged by this permutation. In the second line there are N natural numbers of the range from 1 up to N, separated by a space, that define a permutation — the numbers P(1), P(2),…, P(N).

Output

You should write an only natural number to the standard output, that is an order of the permutation. You may consider that an answer shouldn't exceed 109.

Sample Input

5
4 1 5 2 3

Sample Output

6

Source

Ural State University Internal Contest October'2000 Junior Session


解題思路:一個數在置換過程中不斷變化,進過若干步之後肯定會變成原來的值,所以我們求出每個數置換爲原來的值得循環節,然後對這些循環節求一個最小公倍數就行。


#include <iostream>
#include <stdio.h>
#include <algorithm>
#include <cstring>
using namespace std;
typedef long long LL;
const int maxn = 1000 + 10;
int N;
int p[maxn];
int a[maxn];
int cir[maxn];
LL gcd(LL x, LL y)
{
    return x == 0 ? y : gcd(y % x, x);
}
LL lcm(LL x, LL y)
{
    return (x * y) / gcd(x, y);
}
int main()
{
    scanf("%d", &N);
    for(int i = 1; i <= N; i++)
    {
        scanf("%d", &p[i]);
        a[i] = i;
    }
    for(int i = 1; i <= N; i++)
    {
        int ans = 1;
        while(p[a[i]] != i)
        {
            a[i] = p[a[i]];
            ans++;
        }
        cir[i] = ans;
    }
    LL ans = 1;
    for(int i = 1; i <= N; i++)
    {
        ans = lcm(ans, (LL)cir[i]);
    }
    printf("%lld\n", ans);
    return 0;
}



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