415. Add Strings（LeetCode）

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.

 1 class Solution {
 2 public:
 3     string addStrings(string num1, string num2) {
 4         if (num1.size() < num2.size()) return addStrings(num2, num1);
 5         int extra = 0;
 6         int idx1 = num1.size() - 1;
 7         int idx2 = num2.size() - 1;
 8         while (idx1 >= 0) {
 9             int sum = extra + num1[idx1] - '0';
10             if (idx2 >= 0)
11                 sum = sum + num2[idx2] - '0';
12             num1[idx1] = '0' + sum % 10;
13             extra = sum / 10;
14             idx1--;
15             idx2--;
16         }
17         if (extra) num1 = "1" + num1;
18         return num1;
19     }
20 };