Letter Case Permutation — LeetCode 784 Python Solution

The problem

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

Example

Input

s = "a1b2"

Output

["a1b2","a1B2","A1b2","A1B2"]

Python solution

class Solution:
    def letterCasePermutation(self, s: str) -> List[str]:
        def dfs(i: int) -> None:
            if i >= len(t):
                ans.append("".join(t))
                return
            dfs(i + 1)
            if t[i].isalpha():
                t[i] = chr(ord(t[i]) ^ 32)
                dfs(i + 1)

        t = list(s)
        ans = []
        dfs(0)
        return ans

Complexity

Pattern: Backtracking

Build candidates one choice at a time and abandon a branch the moment it cannot work. LeetCode 784. Letter Case Permutation is filed here on both counts: the reference solution below belongs to the algorithm family this hub collects, and LeetCode tags it Backtracking.

The backtracking guide has the Python template for the pattern and the 105 LeetCode problems that use it.

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