Your question is Sum of Very Large Integers. Start with the requirements on the right.
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Wayfair pricing and promotion services may need to combine values represented as strings when the numbers exceed native integer limits. Given two integer strings, return their exact sum without converting either complete input to an integer type.
Implement add_strings(num1, num2), where both inputs are non-empty strings representing signed decimal integers. An optional leading - is allowed. Positive signs are not included. Inputs may contain leading zeros. Return a string containing the canonical decimal representation of the sum, with no leading zeros unless the result is zero.
Your algorithm must perform arithmetic manually, digit by digit. Do not use Python's int, arbitrary-precision integer conversion, floating-point arithmetic, or equivalent libraries on the complete inputs.
def add_strings(num1, num2):