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Solution: Letter Combinations of a Phone Number

Explore the backtracking method to solve the letter combinations problem from digit strings. Understand how to use recursion and subsets pattern to generate all valid letter sequences linked to phone keypad digits. This lesson guides you through the implementation, complexity analysis, and recursive approach to build combinations step-by-step.

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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] ...