<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:
    string numberToWords(int num) {
        if (num == 0) return "Zero";
        
        string bigString[] = {"Thousand", "Million", "Billion"};
        string result = numberToWordsHelper(num % 1000);
        num /= 1000;

        for (int i = 0; i < 3; ++i) {
            if (num > 0 && num % 1000 > 0) {
                result = numberToWordsHelper(num % 1000) + bigString[i] + " " + result;
            }
            num /= 1000;
        }

        return result.empty() ? result : result.substr(0, result.size() - 1); // Trim trailing space
    }

    string numberToWordsHelper(int num) {
        string digitString[] = {"Zero", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine"};
        string teenString[] = {"Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
        string tenString[] = {"", "", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};

        string result = "";
        if (num > 99) {
            result += digitString[num / 100] + " Hundred ";
        }
        num %= 100;
        if (num < 20 && num > 9) {
            result += teenString[num - 10] + " ";
        } else {
            if (num >= 20) {
                result += tenString[num / 10] + " ";
            }
            num %= 10;
            if (num > 0) {
                result += digitString[num] + " ";
            }
        }
        
        return result;
    }
};
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