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
func multiply(num1 string, num2 string) string {
	if num1 == "0" || num2 == "0" {
		return "0"
	}
	m, n := len(num1), len(num2)
	arr := make([]int, m+n)
	for i := m - 1; i >= 0; i-- {
		a := int(num1[i] - '0')
		for j := n - 1; j >= 0; j-- {
			b := int(num2[j] - '0')
			arr[i+j+1] += a * b
		}
	}
	for i := len(arr) - 1; i > 0; i-- {
		arr[i-1] += arr[i] / 10
		arr[i] %= 10
	}
	i := 0
	if arr[0] == 0 {
		i = 1
	}
	ans := []byte{}
	for ; i < len(arr); i++ {
		ans = append(ans, byte('0'+arr[i]))
	}
	return string(ans)
}