<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:
    // Arrays to store words for numbers less than 10, 20, and 100
    const vector<string> belowTen = { "", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine" };
    const vector<string> belowTwenty = { "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen" };
    const vector<string> belowHundred = { "", "Ten", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety" };


    string numberToWords(int num) {
        if(num==0)
            return "Zero";
        return convertToWords(num);
    }

    string convertToWords(int num) {
        if (num < 10) {
            return belowTen[num];
        }
        if (num < 20) {
            return belowTwenty[num - 10];
        }
        if (num < 100) {
            return belowHundred[num / 10] + (num % 10 ? " " + convertToWords(num % 10) : "");
        }
        if (num < 1000) {
            return convertToWords(num / 100) + " Hundred" + (num % 100 ? " " + convertToWords(num % 100) : "");
        }
        if (num < 1000000) {
            return convertToWords(num / 1000) + " Thousand" + (num % 1000 ? " " + convertToWords(num % 1000) : "");
        }
        if (num < 1000000000) {
            return convertToWords(num / 1000000) + " Million" + (num % 1000000 ? " " + convertToWords(num % 1000000) : "");
        }
        return convertToWords(num / 1000000000) + " Billion" + (num % 1000000000 ? " " + convertToWords(num % 1000000000) : "");
    }
};
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