# 784. Letter Case Permutation (easy)
###### tags: `online_judge`, `python3`

熱身 easy 題目。各個字母都有大、小寫兩種可能。以 recursive 展開即可，以本題的複雜度來說，改用迴圈也不是不行。只是...我懶。

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### 題目

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 between 1 and 12.
    S will consist only of letters or digits.

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### 解答

class Solution:
    
    ans = []
    
    def letterCasePermutation(self, S: str) -> List[str]:
        self.ans = []
        self.dfs(S,0)
        return self.ans
        
    def dfs(self, input,start_index) -> List[str]:
        
        for i in range(start_index,len(input)):
            if input[i].isalpha():
                
                input = input[:i] + input[i].lower() + input[i+1:]
                self.dfs(input,i+1)

                input = input[:i] + input[i].upper() + input[i+1:]
                self.dfs(input,i+1)

                return
                
        self.ans.append(input)
        return

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### 成績

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