Add Strings
<p>Given two non-negative integers, <code>num1</code> and <code>num2</code> represented as string, return <em>the sum of</em> <code>num1</code> <em>and</em> <code>num2</code> <em>as a string</em>.</p>

<p>You must solve the problem without using any built-in library for handling large integers (such as <code>BigInteger</code>). You must also not convert the inputs to integers directly.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> num1 = &quot;11&quot;, num2 = &quot;123&quot;
<strong>Output:</strong> &quot;134&quot;
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> num1 = &quot;456&quot;, num2 = &quot;77&quot;
<strong>Output:</strong> &quot;533&quot;
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> num1 = &quot;0&quot;, num2 = &quot;0&quot;
<strong>Output:</strong> &quot;0&quot;
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= num1.length, num2.length &lt;= 10<sup>4</sup></code></li>
	<li><code>num1</code> and <code>num2</code> consist of only digits.</li>
	<li><code>num1</code> and <code>num2</code> don&#39;t have any leading zeros except for the zero itself.</li>
</ul>
Math; String; Simulation
class Solution {
    fun addStrings(num1: String, num2: String): String {
        val result = mutableListOf<Int>()

        val chars_1 = num1.toCharArray()
        val chars_2 = num2.toCharArray()
        var over = 0
        var i = num1.length
        var j = num2.length

        while (i > 0 || j > 0 || over > 0) {
            val a = if (i > 0) chars_1[--i] - '0' else 0
            val b = if (j > 0) chars_2[--j] - '0' else 0
            val sum = a + b + over
            over = sum / 10
            result.add(sum % 10)
        }

        return result.reversed().joinToString("")
    }
}
