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Grokking the Coding Interview Patterns in C++

Solution: Letter Combinations of a Phone Number

Explore how to generate all possible letter combinations from digits on a phone number using the subsets pattern and backtracking technique. Understand how to recursively build combinations, handle base cases, and backtrack to explore all valid options. This lesson helps you implement a structured approach to solve combination problems efficiently.

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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] ...