Given a 3 x n board, find the number of ways to fill it with 2 x 1 dominoes.
Example 1:
Following are all the 3 possible ways to fill up a 3 x 2 board.
Example 2:
Here is one possible way of filling a 3 x 8 board. You have to find all the possible ways to do so.
Examples :
Input : 2 Output : 3 Input : 8 Output : 153 Input : 12 Output : 2131
Defining Subproblems:
At any point while filling the board, there are three possible states that the last column can be in:
An = No. of ways to completely fill a 3 x n board. (We need to find this) Bn = No. of ways to fill a 3 x n board with top corner in last column not filled. Cn = No. of ways to fill a 3 x n board with bottom corner in last column not filled.
Note: The following states are impossible to reach:
Finding Recurrences
Note: Even though Bn and Cn are different states, they will be equal for same ‘n’. i.e Bn = Cn
Hence, we only need to calculate one of them.
Calculating An:
Calculating Bn:
The final Recursive Relations are:
Base Cases:
// C++ program to find no. of ways // to fill a 3xn board with 2x1 dominoes. #include <iostream> using namespace std;
int countWays( int n)
{ int A[n + 1], B[n + 1];
A[0] = 1, A[1] = 0, B[0] = 0, B[1] = 1;
for ( int i = 2; i <= n; i++) {
A[i] = A[i - 2] + 2 * B[i - 1];
B[i] = A[i - 1] + B[i - 2];
}
return A[n];
} int main()
{ int n = 8;
cout << countWays(n);
return 0;
} |
// Java program to find no. of ways // to fill a 3xn board with 2x1 dominoes. import java.io.*;
class GFG {
static int countWays( int n)
{
int []A = new int [n+ 1 ];
int []B = new int [n+ 1 ];
A[ 0 ] = 1 ; A[ 1 ] = 0 ;
B[ 0 ] = 0 ; B[ 1 ] = 1 ;
for ( int i = 2 ; i <= n; i++)
{
A[i] = A[i - 2 ] + 2 * B[i - 1 ];
B[i] = A[i - 1 ] + B[i - 2 ];
}
return A[n];
}
// Driver code
public static void main (String[] args)
{
int n = 8 ;
System.out.println(countWays(n));
}
} // This code is contributed by anuj_67. |
# Python 3 program to find no. of ways # to fill a 3xn board with 2x1 dominoes. def countWays(n):
A = [ 0 ] * (n + 1 )
B = [ 0 ] * (n + 1 )
A[ 0 ] = 1
A[ 1 ] = 0
B[ 0 ] = 0
B[ 1 ] = 1
for i in range ( 2 , n + 1 ):
A[i] = A[i - 2 ] + 2 * B[i - 1 ]
B[i] = A[i - 1 ] + B[i - 2 ]
return A[n]
n = 8
print (countWays(n))
# This code is contributed by Smitha |
// C# program to find no. of ways // to fill a 3xn board with 2x1 dominoes. using System;
class GFG {
static int countWays( int n)
{
int []A = new int [n+1];
int []B = new int [n+1];
A[0] = 1; A[1] = 0;
B[0] = 0; B[1] = 1;
for ( int i = 2; i <= n; i++)
{
A[i] = A[i - 2] + 2 * B[i - 1];
B[i] = A[i - 1] + B[i - 2];
}
return A[n];
}
// Driver code
public static void Main ()
{
int n = 8;
Console.WriteLine(countWays(n));
}
} // This code is contributed by anuj_67. |
<script> // Javascript program to find no. of ways
// to fill a 3xn board with 2x1 dominoes.
function countWays(n)
{
let A = new Array(n+1);
let B = new Array(n+1);
A[0] = 1; A[1] = 0;
B[0] = 0; B[1] = 1;
for (let i = 2; i <= n; i++)
{
A[i] = A[i - 2] + 2 * B[i - 1];
B[i] = A[i - 1] + B[i - 2];
}
return A[n];
}
let n = 8;
document.write(countWays(n));
// This code is contributed by rameshtravel07.
</script> |
<?php // PHP program to find no. of ways // to fill a 3xn board with 2x1 dominoes. function countWays( $n )
{ $A = array ();
$B = array ();
$A [0] = 1; $A [1] = 0;
$B [0] = 0; $B [1] = 1;
for ( $i = 2; $i <= $n ; $i ++)
{
$A [ $i ] = $A [ $i - 2] + 2 *
$B [ $i - 1];
$B [ $i ] = $A [ $i - 1] +
$B [ $i - 2];
}
return $A [ $n ];
} // Driver Code $n = 8;
echo countWays( $n );
// This code is contributed by anuj_67. ?> |
Output
153
Time Complexity: O(N)
Auxiliary Space: O(N)