LeetCode in Kotlin

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

class Solution {
    fun addStrings(num1: String, num2: String): String {
        var endNum1 = num1.length - 1
        var endNum2 = num2.length - 1
        val res = StringBuilder()
        var carry = 0
        var sum: Int
        while (endNum1 >= 0 || endNum2 >= 0) {
            val a = if (endNum1 >= 0) num1[endNum1] - '0' else 0
            val b = if (endNum2 >= 0) num2[endNum2] - '0' else 0
            sum = (a + b + carry) % 10
            carry = (a + b + carry) / 10
            res.append(sum)
            endNum1--
            endNum2--
        }
        if (carry != 0) {
            res.append(carry)
        }
        return res.reverse().toString()
    }
}

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