R7-回溯篇
class Solution:def letterCombinations(self, digits: str) -> List[str]:if not digits:return []phone={'2':['a','b','c'],'3':['d','e','f'],'4':['g','h','i'],'5':['j','k','l'],'6':['m','n','o'],'7':['p','q','r','s'],'8':['t','u','v'],'9':['w','x','y','z']}def backtrack(conbination,nextdigit):if len(nextdigit)==0:ret.append(conbination)else:for letter in phone[nextdigit[0]]:backtrack(conbination+letter,nextdigit[1:])ret=[]backtrack('',digits)return ret 
