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Grokking the Coding Interview Patterns in Python

Letter Combinations of a Phone Number

Explore techniques to generate all possible letter combinations from digits on a phone keypad, focusing on understanding digit-to-letter mappings and implementing efficient subset solutions in Python. This lesson helps you develop skills critical for solving combinatorial coding interview problems.

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Statement

Given a string containing digits from 2 to 9 inclusive, with the possibility of each digit appearing multiple times, return all possible letter combinations that the number could represent. Return the answer in any order.

The illustration below shows the mapping of digits to letters in a telephone dial pad.

Note: The number 11 on the telephone dial pad does not correspond to any letter, so the input string only contains digits from 22 to 99.

Constraints:

  • 0≤0 \leq digits.length ≤4\leq 4

  • digits[i] is a digit in the range [2,9][2, 9]

Examples

Understand the problem

Let’s take a moment to make sure you’ve correctly understood the problem. The quiz below helps you check if you’re solving the correct problem:

Letter Combinations of a Phone Number

1.

What should be the output if the following digits are given as input?

Digits = 29

A.

[aw, ax, ay, az]

B.

[bw, bx, by, bz]

C.

[aw, ax, ay, az, bw, bx, by, bz, cw, cx, cy, cz]

D.

[cw, cx, cy, cz]

1 / 4

Figure it out!

We have a game for you to play. Rearrange the logical building blocks to develop a clearer understanding of how to solve this problem.

Sequence - Vertical
Drag and drop the cards to rearrange them in the correct sequence.
1
2
3
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5

Try it yourself

Implement your solution in the following coding playground:

Python
usercode > main.py
def letter_combinations(digits):

    # Replace this placeholder return statement with your code

    

    return []
Letter Combinations of a Phone Number