题目描述：

LeetCode 784. Letter Case Permutation

Given a string S, we can transform every letter individually to be lowercase or uppercase to create another string.  Return a list of all possible strings we could create.

Examples:
Input: S = "a1b2"
Output: ["a1b2", "a1B2", "A1b2", "A1B2"]

Input: S = "3z4"
Output: ["3z4", "3Z4"]

Input: S = "12345"
Output: ["12345"]

Note:

• S will be a string with length at most 12.
• S will consist only of letters or digits.

题目大意：

给定字符串S，将其中的字母分别转化为大、小写，求所有组合。

解题思路：

递归（Recursion）

Python代码：

class Solution(object):
    def letterCasePermutation(self, S):
        """
        :type S: str
        :rtype: List[str]
        """
        if not S: return [S]
        rest = self.letterCasePermutation(S[1:])
        if S[0].isalpha():
            return [S[0].lower() + s for s in rest] + [S[0].upper() + s for s in rest]
        return [S[0] + s for s in rest]

 

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