<p>Convert a non-negative integer <code>num</code> to its English words representation.</p>

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

<pre>
<strong>Input:</strong> num = 123
<strong>Output:</strong> &quot;One Hundred Twenty Three&quot;
</pre>

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

<pre>
<strong>Input:</strong> num = 12345
<strong>Output:</strong> &quot;Twelve Thousand Three Hundred Forty Five&quot;
</pre>

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

<pre>
<strong>Input:</strong> num = 1234567
<strong>Output:</strong> &quot;One Million Two Hundred Thirty Four Thousand Five Hundred Sixty Seven&quot;
</pre>

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

<ul>
	<li><code>0 &lt;= num &lt;= 2<sup>31</sup> - 1</code></li>
</ul>

class Solution {
public:
    const vector<string> belowTwenty{"", "One", "Two", "Three", "Four","Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
    const vector<string> tens{"", "", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};
    string numberToWords(int num) {
        if (num == 0) return "Zero";
        return helper(num).substr(1); // Remove the leading space
    }

private:
    string helper(int num) {
        if (num >= 1000000000) return helper(num / 1000000000) + " Billion" + helper(num % 1000000000);
        if (num >= 1000000) return helper(num / 1000000) + " Million" + helper(num % 1000000);
        if (num >= 1000) return helper(num / 1000) + " Thousand" + helper(num % 1000);
        if (num >= 100) return helper(num / 100) + " Hundred" + helper(num % 100);
        if (num >= 20) return " " + tens[num / 10] + helper(num % 10);
        if (num >= 1) return " " + belowTwenty[num];
        return "";
    }
};
12300