Given two non-negative integers, num1 and num2 represented as string, return the sum of num1 and num2 as a string. Example 1: Input: num1 = "11", num2 = "123" Output: "134" Example 2: Input: num1 = "456", num2 = "77" Output: "533" Example 3: Input: num1 = "0", num2 = "0" Output: "0" Constraints: 1 <= num1.length, num2.length <= 104 num1 and num2 consist of only digits. num1 and num2 don't have any leading zeros except for the zero itself. Follow up: Could you solve it without using any built-in BigInteger library or converting the inputs to integer directly?