415. Add Strings

// LeetCode URL: https://leetcode.com/problems/add-strings/
/**
 * Time Complexity: O(max(M, N))
 *
 * Space Complexity: O(max(M, N))
 *
 * M = Length of num1 string. N = Length of num2 string.
 */
class Solution {
    public String addStrings(String num1, String num2) {
        if (num1 == null || num2 == null) {
            return "";
        }
        if (num1.length() == 0) {
            return num2;
        }
        if (num2.length() == 0) {
            return num1;
        }

        StringBuilder sb = new StringBuilder();
        int i = num1.length() - 1;
        int j = num2.length() - 1;
        int carry = 0;

        while (i >= 0 || j >= 0 || carry > 0) {
            int x = i < 0 ? 0 : num1.charAt(i) - '0';
            int y = j < 0 ? 0 : num2.charAt(j) - '0';
            int sum = x + y + carry;
            sb.append(sum % 10);
            carry = sum / 10;
            i--;
            j--;
        }

        return sb.reverse().toString();
    }
}