784. Letter Case Permutation

package LeetCode_784

/**
 * 784. Letter Case Permutation
 * https://leetcode.com/problems/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. 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:
1. S will be a string with length between 1 and 12.
2. S will consist only of letters or digits.
 * */
class Solution {
    /*
    * solution: dfs + backtracking, Time:O(2^n), Space:O(2^n)
    * */
    fun letterCasePermutation(S: String): List<String> {
        val result = ArrayList<String>()
        if (S == null || S.isEmpty()) {
            return result
        }
        dfs(0, S.toCharArray(), result)
        return result
    }

    private fun dfs(pos: Int, chars: CharArray, result: ArrayList<String>) {
        if (pos == chars.size) {
            result.add(String(chars))
            return
        }
        if (chars[pos].isDigit()) {
            //if digit, check next char
            return dfs(pos + 1, chars, result)
        }
        //change current pos to lower case
        chars[pos] = chars[pos].toLowerCase()
        //and keep tracking via next level
        dfs(pos + 1, chars, result)
        //change current pos to upper case
        chars[pos] = chars[pos].toUpperCase()
        //and keep tracking via next level
        dfs(pos + 1, chars, result)
    }
}