Problem 1: Add String Numbers
Given two non-negative integers num1 and num2 represented as string, return the sum of num1 and num2. You can not use any built-in BigInteger library or convert the inputs to integer directly.
Example 1
Input: num1 = "123", num2 = "456"
Output: 579
Problem 2: Case Permutations
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.
String will only contain letters and numbers
Example 1
Input: "a1b2"
Output: ["a1b2","a1B2","A1b2","A1B2"]
Example 2
Input: "1"
Output: ["1"]
