Letter Case Permutation

Description

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. You can return the output in any order.

 

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"]

Example 4:

Input: S = "0"
Output: ["0"]

 

Constraints:

• S will be a string with length between 1 and 12.
• S will consist only of letters or digits.

Solution(javascript)

const letterCasePermutation = function (S = '') {
  const result = []
  const aux = (index = 0, current = '') => {
    if (index >= S.length) {
      result.push(current)
      return
    }
    if (/[a-zA-Z]/.test(S[index])) {
      aux(index + 1, current + S[index].toLowerCase())
      aux(index + 1, current + S[index].toUpperCase())
    } else {
      aux(index + 1, current + S[index])
    }
  }
  aux()
  return result
}