Letter case permutation

Time: O(Nx2^N); Space: O(1); easy

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.

Example 1:

Input: S = “a1b2”

Output: [“a1b2”, “a1B2”, “A1b2”, “A1B2”]

Example 2:

Input: S = “3z4”

Output: [“3z4”, “3Z4”]

Example 3:

Input: S = “12345”

Output: [“12345”]

Constraints:

  • S will be a string with length between 1 and 12.

  • S will consist only of letters or digits.

[1]:

class Solution1(object):
    """
    Time: O(N*2^N)
    Space: O(N*2^N)
    """
    def letterCasePermutation(self, S):
        """
        :type S: str
        :rtype: List[str]
        """
        result = [[]]
        for c in S:
            if c.isalpha():
                for i in range(len(result)):
                    result.append(result[i][:])
                    result[i].append(c.lower())
                    result[-1].append(c.upper())
            else:
                for s in result:
                    s.append(c)

        return list(map(''.join, result))

[4]:

sol = Solution1()
S = "a1b2"
# print(sol.letterCasePermutation(S))
assert sol.letterCasePermutation(S) == ['a1b2', 'A1b2', 'a1B2', 'A1B2']
S = "3z4"
assert sol.letterCasePermutation(S) == ["3z4", "3Z4"] or ["3Z4", "3z4"]
S = "12345"
assert sol.letterCasePermutation(S) == ["12345"]