LeetCode 415. Add Strings

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

Example 1:

Input: num1 = "11", num2 = "123"
Output: "134"

Example 2:

Input: num1 = "456", num2 = "77"
Output: "533"

Example 3:

Input: num1 = "0", num2 = "0"
Output: "0"

Constraints:

• 1 <= num1.length, num2.length <= 104

• num1 and num2 consist of only digits.

• num1 and num2 don't have any leading zeros except for the zero itself.

Follow up: Could you solve it without using any built-in BigInteger library or converting the inputs to integer directly?

Solution

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class Solution {
public:
    string addStrings(string num1, string num2) {
        string result;
        int carry = 0;
        int i1 = num1.length() - 1;
        int i2 = num2.length() - 1;
        while (i1 >= 0 || i2 >= 0 || carry > 0) {
            int v1 = i1 >= 0 ? num1[i1] - '0' : 0;
            int v2 = i2 >= 0 ? num2[i2] - '0' : 0;
            int sum = v1 + v2 + carry;
            result.push_back(sum % 10 + '0');
            carry = sum / 10;
            i1--;
            i2--;
        }
        
        reverse(begin(result), end(result));
        return result;
    }
};