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Java-based LeetCode algorithm problem solutions, regularly updated

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package g0401_0500.s0415_add_strings;

// #Easy #String #Math #Simulation #Data_Structure_II_Day_6_String
// #2022_07_16_Time_3_ms_(82.41%)_Space_43.1_MB_(66.56%)

/**
 * 415 - Add Strings\.
 *
 * Easy
 *
 * Given two non-negative integers, `num1` and `num2` represented as string, return _the sum of_ `num1` _and_ `num2` _as a string_.
 *
 * You must solve the problem without using any built-in library for handling large integers (such as `BigInteger`). You must also not convert the inputs to integers directly.
 *
 * **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.
**/
public class Solution {
    public String addStrings(String num1, String num2) {
        StringBuilder result = new StringBuilder();
        int carry = 0;
        for (int i = num1.length() - 1, j = num2.length() - 1;
                i >= 0 || j >= 0 || carry != 0;
                i--, j--) {
            int sum = carry;
            if (i >= 0) {
                sum += Character.digit(num1.charAt(i), 10);
            }
            if (j >= 0) {
                sum += Character.digit(num2.charAt(j), 10);
            }
            carry = sum / 10;
            result.append(sum % 10);
        }
        return result.reverse().toString();
    }
}