There is a standard numeric keypad on a mobile phone. You can only press the current button or buttons that are directly up, left, right, or down from it (for ex if you press 5, then pressing 2, 4, 6 & 8 are allowed). Diagonal movements and pressing the bottom row corner buttons (* and #) are prohibited.Given a number n, find the number of possible unique sequences of length n that you can create by pressing buttons. You can start from any digit.

This problem can be solved using dynamic programming. Here are the steps:

1. First, we need to define the keypad layout. We can represent it as a 2D array:

   int keypad[4][3] = {{1, 2, 3},
                       {4, 5, 6},
                       {7, 8, 9},
                       {-1, 0, -1}};
   

   Here, -1 represents the * and # keys which are not allowed.

2. Next, we need to define a 3D array to store the number of sequences of a given length starting from a particular key. The dimensions of the array will be 10 (for the 10 digits), n+1 (for sequence lengths from 0 to n), and 5 (for the 5 possible moves - stay, up, down, left, right). Initialize this array to -1 to indicate that we haven't computed any values yet.

3. Now, we define a recursive function that takes the current position (i, j) on the keypad, the remaining length of the sequence, and the 3D array as input. This function will return the number of sequences of the given length starting from the current key.

4. In this function, first we check if the remaining length is 0. If it is, we return 1 because there is exactly one sequence of length 0 (the empty sequence).

5. If the remaining length is not 0, we check if we have already computed the number of sequences for the current key and remaining length. If we have, we return the computed value.

6. If we haven't computed the value yet, we calculate it by making all possible moves from the current key and recursively calling the function for the new key and remaining length minus 1. We add up all these values to get the total number of sequences.

7. Finally, we store the computed value in the 3D array and return it.

8. To get the total number of sequences of length n, we call the function for each key on the keypad and add up the results.

This algorithm runs in O(n) time and uses O(n) space.