38.字符串的排列

写这段代码的时候就是不知道在哪里写递归函数，一开始写的是res.append(head+self.Permutation(shadow)),果然递归就是一看就会一写就废

class Solution:
    def Permutation(self, ss):
        # write code here
        if len(ss) == 0:
            return []
        if len(ss) == 1:
            return ss
        res = []
        for i in range(len(ss)):
            head = ss[i]
            shadow = ss[:i]+ss[i+1:]
            for j in self.Permutation(shadow):
                res.append(head + j)
        return sorted(list(set(res)))

• 一种更好理解的方法：

  class Solution:
    def Permutation(self, ss):
        # write code here
        if not ss:
            return []
        res = []
        def backtrack(nums, tmp):
            if not nums:
                res.append(tmp)
                return 
            for i in range(len(nums)):
                backtrack(nums[:i] + nums[i+1:], tmp + nums[i])
        backtrack(ss, '')
        return sorted(list(set(res)))

leetcode两道道相似的题：
（全排列）https://leetcode-cn.com/problems/permutations/
（电话号码的组合）：https://leetcode-cn.com/problems/letter-combinations-of-a-phone-number/