415. Add Strings

Given two non-negative integers 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.

这个题就是把两个字符串加起来，我自己归类到“部分开放字符串问题”，因为目标字符串的长度不确定，所以可以从后向前计算。详情请参见482. License Key Formatting

class Solution {
public:
    string addStrings(string num1, string num2) {
        if (num1.length() ==0) return num2;
        if (num2.length() == 0) return num1;
        int index1 = num1.length() - 1;
        int index2 = num2.length() - 1;
        string result;
        int carry = 0;
        while (index1 >= 0 || index2 >= 0) {
            int d1 = index1 >= 0 ? num1[index1--] - '0' : 0;
            int d2 = index2 >= 0 ? num2[index2--] - '0' : 0;
            int sum = d1 + d2 + carry;
            result += ((sum % 10) + '0');
            carry = sum / 10;
          
        }
        if (carry != 0) result += (carry + '0');
        return string(result.rbegin(), result.rend());
    }
};