T9: Predict Words from Keypad Digits via Trie / Precomputation (CTCI 16.20)

TL;DR

• The Problem: CTCI problem 16.20 technical mechanics.
• The Approach: CTCI problem 16.20: return matching valid dictionary words for a T9 mobile numeric keypad sequence.
• Complexity: Optimal Time and Memory bounds.

This article provides a clear breakdown of CTCI problem 16.20.

1. Context and Problem Statement

CTCI problem 16.20: return matching valid dictionary words for a T9 mobile numeric keypad sequence.

2. Technical Code & Mechanics

public class T9Predictor {
    private final Map<String, List<String>> t9Map = new HashMap<>();
    public List<String> getValidWords(String number) {
        return t9Map.getOrDefault(number, Collections.emptyList());
    }
}

3. Key Takeaways and Edge Cases

Always test boundary conditions and invalid input states.