Hint: !

To me there is a typo in the following sentence from the problem:

"We can have encryption schemes where 1 can be mapped to 3 or 2 or 1, 2 can be mapped to the remaining two, and 1 can be mapped to the unmapped one."

I think the last instance of "1" should be "3" so that it reads thusly:

"We can have encryption schemes where 1 can be mapped to 3 or 2 or 1, 2 can be mapped to the remaining two, and 3 can be mapped to the unmapped one."

Does anyone else agree?

A simple factorial calculator.

All you have to calculate is factorial

import java.io.; import java.util.; import java.text.; import java.math.; import java.util.regex.*;

public class Solution {

public static void main(String[] args) {
    /* Compute n! using iteration*/
    Scanner sc = new Scanner (System.in);

    int n = sc.nextInt();
    sc.close();
    int ans = 1;
    for (int i = 1; i <= n; i++) {
        ans*=i;
    } 
    System.out.println(ans);
}

}

/In c it so simple a basic program to do./

include

include

include

include

int main() { int i,n,result; scanf("%d",&n); for(i=0;i

Not many of these solutions work for large n ... and cryptography is full of large ns. A real-world input to this problem might have M and C representing the possible blocks of say 8 bytes - what's the size of the set of all 64-bit numbers? 18446744073709551616. Number of bijections? $VERYBIG.

Try even 1024 as input ...