python 784. 字母大小写全排列--每日一题

class Solution:
    def letterCasePermutation(self, S: str) -> List[str]:
        L=[]
        for i in range(len(S)):
            if L==[]:
                L.append(S)
            if S[i].isalpha():
                lenl=len(L)
                for x in range(lenl):
                    Lx=list(L[x])
                    if Lx[i].islower():
                        xl=[a for a in Lx]
                        xl[i]=xl[i].upper()
                        L.append("".join(xl))
                    else:
                        xl = [a for a in Lx]
                        xl[i] = xl[i].lower()
                        L.append("".join(xl))
            else:
                continue
        return L