415. Add Strings

Given two non-negative integers num1 and num2 represented as string, return the sum of num1 and num2.


  1. The length of both num1 and num2 is < 5100.
  2. Both num1 and num2 contains only digits 0-9.
  3. Both num1 and num2 does not contain any leading zero.
  4. You must not use any built-in BigInteger library or convert the inputs to integer directly.