public static int [] randomPerm(int len){
int [] perm=new int[len];
Random generator = new Random();
for (int i=0; i<len; i++){
perm[i]=i+1;
}
for (int i=0; i<len; i++){
int offset = generator.nextInt(len-i);
swap(perm,i,i+offset);
}
return perm;
}