<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>

#include <string>
using namespace std;

class Solution {
    string belowten[10] = {"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine"};
    string belowtwenty[10] = {"Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
    string belowhundred[10] = {"", "Ten", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};
    
public:
    string numberToWords(int num) {
        if(num == 0) return "Zero";
        return helper(num);
    }
    
private:
    string helper(int num) {
        if(num < 10) return belowten[num];
        if(num < 20) return belowtwenty[num-10];
        if(num < 100) return belowhundred[num/10] + (num%10 != 0 ? " " + belowten[num%10] : "");
        if(num < 1000) return belowten[num/100] + " Hundred" + (num%100 != 0 ? " " + helper(num%100) : "");
        if(num < 1000000) return helper(num/1000) + " Thousand" + (num%1000 != 0 ? " " + helper(num%1000) : "");
        if(num < 1000000000) return helper(num/1000000) + " Million" + (num%1000000 != 0 ? " " + helper(num%1000000) : "");
        return helper(num/1000000000) + " Billion" + (num%1000000000 != 0 ? " " + helper(num%1000000000) : "");
    }
};

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