415. Add Strings（LeetCode）

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.

   class Solution {
   public:
       string addStrings(string num1, string num2) {
           if (num1.size() < num2.size()) return addStrings(num2, num1);
           int extra = 0;
           int idx1 = num1.size() - 1;
           int idx2 = num2.size() - 1;
           while (idx1 >= 0) {
               int sum = extra + num1[idx1] - '0';
               if (idx2 >= 0)
                   sum = sum + num2[idx2] - '0';
               num1[idx1] = '0' + sum % 10;
               extra = sum / 10;
               idx1--;
               idx2--;
           }
           if (extra) num1 = "1" + num1;
           return num1;
       }
   };