415_add_strings.rb

# Leetcode Problem: https://leetcode.com/problems/add-strings/
#Given two non-negative integers num1 and num2 represented as string, return the sum of num1 and num2.
#
# Note:
#
#     The length of both num1 and num2 is < 5100.
#     Both num1 and num2 contains only digits 0-9.
#     Both num1 and num2 does not contain any leading zero.
#     You must not use any built-in BigInteger library or convert the inputs to integer directly.
# ----------------------------------------------------------------------------------------------------------------------
# @param {String} num1
# @param {String} num2
# @return {String}
def add_strings(num1, num2)
  sum = ''
  carry = 0
  while num1.length.positive? || num2.length.positive?
    last_num1 = num1[0.-1]
    num1 = num1.chomp(last_num1) unless last_num1.nil?

    last_num2 = num2[0.-1]
    num2 = num2.chomp(last_num2) unless last_num2.nil?

    current_sum = last_num1.to_i + last_num2.to_i + carry
    carry = current_sum/10
    sum = (current_sum % 10).to_s + sum
    sum = carry.to_s + sum if num1 == '' && num2 == '' && carry != 0
   end

  sum
end

p add_strings('12', '13')
p add_strings('12', '19')
p add_strings('9', '1')
p add_strings('0', '0')