letterCombinations.py

class Solution:
    
    def letterCombinations(self, digits):
            res = []
            if not digits:
                return []
            digits_to_letters = {
                '2': 'abc',
                '3': 'def',
                '4': 'ghi',
                '5': 'jkl',
                '6': 'mno',
                '7': 'pqrs',
                '8': 'tuv',
                '9': 'wxyz',
            }
            
            def backtrack(idx, comb):
                
                    if idx == len(digits):
                        res.append(comb[:])
                        return
                    
                    for letter in digits_to_letters[digits[idx]]:
                            backtrack(idx+1, comb+letter) 
                  
            backtrack(0, '')
            return res
            
      
myVar = Solution()
myVar.letterCombinations("23")