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 Reccurences**

**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:**

**Final Recursive Relations are:**

**Base Cases:**

## C++

`// 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; ` `} ` |

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## Java

`// 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. ` |

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## Python 3

`# 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 ` |

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## C#

`// 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. ` |

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## PHP

`<?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. ` `?> ` |

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**Output :**

153

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