How should I practice Android Unlock Pattern Count?

Start by reading the full statement, solve the visible examples, then compare the pattern against related Google onsite interview problems in FastPrep.

Is Android Unlock Pattern Count associated with Google?

FastPrep tags this as a Google onsite interview coding practice problem when the problem catalog has that company and stage context.

Android Unlock Pattern Count

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Problem statement

The Android unlock screen has nine keys in a 3x3 grid, numbered:

1 2 3
4 5 6
7 8 9

A pattern visits distinct keys. A move that passes directly through another key is allowed only if that intermediate key was already visited. For example, 1→3 requires 2, 1→9 requires 5, and 3→7 requires 5. All other moves are allowed.

Return the number of valid patterns whose length is between minLength and maxLength, inclusive.

Function

numberOfUnlockPatterns(minLength: int, maxLength: int) → int

Examples

Example 1

minLength = 1maxLength = 1return = 9

Every individual key is a valid length-one pattern.

Example 2

minLength = 1maxLength = 2return = 65

There are 9 length-one patterns and 56 valid length-two patterns.

Example 3

minLength = 4maxLength = 4return = 1624

The standard Android movement rules permit 1624 patterns of exactly four keys.

Constraints

1 <= minLength <= maxLength <= 9.