【剑指offer】字符串的排列

回溯的方法

class Solution:
    def permutation(self, s: str) -> List[str]:
        ans = set()
        def backtrace(path,tmp):
            if path=="":
                ans.add(tmp)
                return
            for i in range(len(path)):
                backtrace(path[:i]+path[i+1:],tmp+ path[i])
        backtrace(s,"")
        return list(ans)