Permutation ii lcci
class Solution:
def permutation(self, S: str) -> List[str]:
n = len(S)
ret = []
if not n: return [""]
for i in range(n):
if S[i] in S[:i]:
continue
for s in self. permutation(S[:i]+S[i+1:]):
ret.append(S[i]+s)
return ret