415.两个字符串相加 Add Strings

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

Note:

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.

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public class Solution {
    public string AddStrings(string num1, string num2) {
        string s = "";
        int maxLength = Math.Max(num1.Length, num2.Length);
        num1 = num1.PadLeft(maxLength, '0');
        num2 = num2.PadLeft(maxLength, '0');
        int carry = 0;
        int digit = 0;
        int i = maxLength - 1;
        while (i >= 0 || carry>0)
        {
            digit = carry;
            if (i >= 0)
            {
                digit += ((int)num1[i] - 48) + ((int)num2[i] - 48);
            }
            if (digit >= 10)
            {
                carry = digit / 10;
            }
            else
            {
                carry = 0;
            }
            s = (digit % 10).ToString() + s;
            i--;
        }
        return s;
    }
}

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