Multiply Strings
<p>Given two non-negative integers <code>num1</code> and <code>num2</code> represented as strings, return the product of <code>num1</code> and <code>num2</code>, also represented as a string.</p>

<p><strong>Note:</strong>&nbsp;You must not use any built-in BigInteger library or convert the inputs to integer directly.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<pre><strong>Input:</strong> num1 = "2", num2 = "3"
<strong>Output:</strong> "6"
</pre><p><strong class="example">Example 2:</strong></p>
<pre><strong>Input:</strong> num1 = "123", num2 = "456"
<strong>Output:</strong> "56088"
</pre>
<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= num1.length, num2.length &lt;= 200</code></li>
	<li><code>num1</code> and <code>num2</code> consist of digits only.</li>
	<li>Both <code>num1</code> and <code>num2</code>&nbsp;do not contain any leading zero, except the number <code>0</code> itself.</li>
</ul>
Math; String; Simulation
class Solution {
    fun multiply(num1: String, num2: String): String {
        if (num1 == "0" || num2 == "0") return "0"

        val chars_1 = num1.toCharArray().reversedArray()
        val chars_2 = num2.toCharArray().reversedArray()

        val result = mutableListOf<Int>()

        chars_1.forEachIndexed { i, c1 ->
            val multiplier_1 = c1 - '0'
            var over = 0
            var index = 0

            fun sum(product: Int = 0): Unit {
                while (index >= result.size) {
                    result.add(0)
                }
                val value = product + over + result[index]
                result[index] = value % 10
                over = value / 10
                return
            }

            chars_2.forEachIndexed { j, c2 ->
                index = i + j
                val multiplier_2 = c2 - '0'
                sum(multiplier_1 * multiplier_2)
            }

            while (over > 0) {
                index++
                sum()
            }
        }

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