text: class Solution:
      def letterCombinations(self, digits: str) -> List[str]:
          di = {"2": "abc", "3": "def", "4":"ghi", "5": "jkl", "6": "mno", "7":"pqrs","8":"tuv", "9":"wxyz"}
          if digits=="":
              return []
          output = [alph for alph in di[digits[0]]]
          # 23
          for digit in digits[1:]:
              cur = []
              while(output):
                  target = output.pop(0)
                  for alph in di[digit]:
                      cur.append(target+alph)
              output.extend(cur)
                      
          return output