UVa 11716 - Digital Fortress

Problem link

Keyword: string, ad hoc, math

Algorithm:

First you have to check the validity of the string.
A string is said to be valid if its length is a perfect square number.
If so, then you can use two-nested for loop to print the grid out.

Code:

#include<cstdio>
#include<cstring>
#include<cmath>
#include<algorithm>
using namespace std;

int main()
{
    char in[ 10001 ];
    int t, len, sq[ 101 ];
    for( int i = 0; i <= 100; ++i )
        sq[ i ] = i*i;
    scanf( "%d\n", &t );

    while( t-- )
    {
        gets( in );
        len = strlen( in );
        if( !binary_search( sq, sq+101, len ) )
            puts( "INVALID" );
        else
        {
            int root = sqrt( len ) + 1e-12;
            for( int i = 0; i < root; ++i )
                for( int j = 0; j < root; ++j )
                    putchar( in[ j*root + i ] );
            puts( "" );
        }
    }

    return 0;
}