My Source:

#include <stdio.h>

int nok(int a,int b)
{
    int a1=a,b1=b;
    while (a!=b)
    {
        if(a>b) a-=b;
        else b-=a;
    }
    return(a1*b1/a);
}

int main()
{
    int n,per[1010];
    int a[1010];

    scanf("%d",&n);
    for(int i=1;i<=n;i++)
    {
        scanf("%d",&per[i]);
        a[i] = 0;
    }

    for(i=1;i<=n;i++)
    {
        if(a[i]==0)
        {
            a[i]=1;
            int pos = per[i];
            while(pos!=i)
            {
                pos = per[pos];
                a[i]++;
            }
            pos = per[i];
            while(pos!=i)
            {
                a[pos] = a[i];
                pos = per[pos];
            }
        }
    }

    for(i=2;i<=n;i++)
    {
        a[i] = nok(a[i-1],a[i]);
    }
    printf("%d",a[n]);

    return(0);
}

"for (int i = 0..." isn't accepted, declare i separately