Daily Coding Problem: Problem #985 [Medium]

Good morning! Here's your coding interview problem for today.

This problem was asked by Uber.

One way to unlock an Android phone is through a pattern of swipes across a 1-9 keypad.

For a pattern to be valid, it must satisfy the following:

• All of its keys must be distinct.
• It must not connect two keys by jumping over a third key, unless that key has already been used.

For example, 4 - 2 - 1 - 7 is a valid pattern, whereas 2 - 1 - 7 is not.

Find the total number of valid unlock patterns of length N, where 1 <= N <= 9.

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