LeetCode-in-Java

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.

Solution

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();
    }
}

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