/*

Convert a non-negative integer num to its English words representation.

Example 1:

Input: num = 123

Output: "One Hundred Twenty Three"

Example 2:

Input: num = 12345

Output: "Twelve Thousand Three Hundred Forty Five"

Tag-Hard

*/

class Solution {

private final String[] belowTwenty = {"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine",

"Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};

private final String[] tens = {"", "", "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) {

StringBuilder result = new StringBuilder();

if (num < 20) {

result.append(belowTwenty[num]);

} else if (num < 100) {

result.append(tens[num / 10]).append(" ").append(belowTwenty[num % 10]);

} else if (num < 1000) {

result.append(helper(num / 100)).append(" Hundred ").append(helper(num % 100));

} else if (num < 1000000) {

result.append(helper(num / 1000)).append(" Thousand ").append(helper(num % 1000));

} else if (num < 1000000000) {

result.append(helper(num / 1000000)).append(" Million ").append(helper(num % 1000000));

} else {

result.append(helper(num / 1000000000)).append(" Billion ").append(helper(num % 1000000000));

}

return result.toString().trim();

}

}