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.…