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Letter Case Permutation

Problem statement​

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. Return the output in any order.

Example 1:

Input: s = "a1b2"Output: ["a1b2","a1B2","A1b2","A1B2"]

Example 2:

Input: s = "3z4"Output: ["3z4","3Z4"]

Constraints:

  • 1 <= s.length <= 12
  • s consists of lowercase English letters, uppercase English letters, and digits.

My solution​

/**
 * @param {string} s
 * @return {string[]}
 */
var letterCasePermutation = function(s) {
    const strs = new Set();
    
    function traverse(currentIndex, str) {
        if (currentIndex === s.length) {
             !strs.has(str) && strs.add(str);
            return;
        }
        
        const char = s.charAt(currentIndex);
        if (typeof char === "number") {
            traverse(currentIndex + 1, `${str}${char}`)
        } else {
            traverse(currentIndex + 1, `${str}${char.toUpperCase()}`)
            traverse(currentIndex + 1, `${str}${char.toLowerCase()}`)
        }
    }
    
    traverse(0, "")
    
    return [...strs]
};