# 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.

class Solution:

def addStrings(self, num1, num2):

if len(num1) < len(num2):

return self.sumNums(num1, num2)

else:

return self.sumNums(num2, num1)

def sumNums(self, low, high):

dict = {'0': 0, '1': 1, '2': 2, '3': 3, '4': 4, '5': 5, '6': 6, '7': 7, '8': 8, '9': 9}

i = len(low) - 1

j = len(high) - 1

res = []

carry = 0

while j >= 0:

while i >= 0:

add = dict[low[i]] + dict[high[j]] + carry

res = [str(add % 10)] + res

carry = add // 10

i -= 1

j -= 1

if j >= 0:

add = dict[high[j]] + carry

carry = add // 10

res = [str(add % 10)] + res

j -= 1

if carry > 0:

res = [str(carry)] + res

return "".join(res)