The problem is simple to understand but great for practicing recursion. Each digit from 2 to 9 represents a set of letters, similar to the keys on a traditional phone keypad. For example, 2 maps to abc and 3 maps to def.

For the input "23", the expected combinations are:

["ad","ae","af","bd","be","bf","cd","ce","cf"]

💡 Approach

I first created a phoneMap object to store the letter mapping for each digit. Then, I used a recursive backtrack() function to build combinations step by step.

At every recursive call:

• Check whether all digits have been processed.
• If yes, add the current combination to the result.
• Otherwise, get the letters corresponding to the current digit.
• Loop through each letter and recursively continue with the next digit.

This approach explores every possible combination systematically and uses the call stack to manage the recursion.

📌 Complexity

If there are n digits and each digit can represent up to 4 letters, the time complexity is O(4ⁿ), with additional space required for recursion and the resulting combinations.

Another great reminder that backtracking is all about choosing, exploring, and then moving forward.

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