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Android Unlock Pattern LLD: Nine Dots, One Skip Table, 389,112 Secrets

A low-level design walkthrough of the Android unlock pattern: the crossing rule as a lookup table, pattern validation in O(1) per move, and counting all valid patterns with backtracking and symmetry.

By fiveyearsdevJune 12, 20268 min read

"Design the Android unlock pattern." Nine dots, a finger, a swoosh — and one of the best-disguised questions in the queue, because it's really two interviews stapled together. The LLD half: encode the pattern rules (where can a finger go next?) so validation is instant. The algorithm half: count every valid pattern — the LeetCode-famous backtracking problem whose answer, 389,112, you can now drop at parties. The hinge between the halves is a single idea this series keeps rediscovering: the rules live on the lines, not the dots — and rules that live on pairs fit in a table. Let's start nowhere …

What’s inside

Let's start nowhere near a computer

Where the pair-table trick runs

Step 1 — Functional requirements (sentences first)

Step 2 — Non-functional requirements

Step 3 — The skip table: geometry, precomputed

Step 4 — Validation: one lookup per hop

Step 5 — The count: backtracking wears the validator

Step 6 — Trade-offs (each one keeping an NFR)

The complete implementation

The interview corner

Where to go from here

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