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28class Solution {
// math: O(1)
private final String[] LESS_THAN_20 = {"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
private final String[] TENS = {"", "Ten", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};
public String numberToWords(int num) {
if (num == 0) return "Zero";
return help(num);
}
String help(int num) {
String ret = "";
if (num < 20) {
ret = LESS_THAN_20[num];
} else if (num < 100) {
ret = TENS[num / 10] + " " + help(num % 10);
} else if (num < 1000) {
ret = help(num / 100) + " Hundred " + help(num % 100);
} else if (num < 1000000) {
ret = help(num / 1000) + " Thousand " + help(num % 1000);
} else if (num < 1000000000) {
ret = help(num / 1000000) + " Million " + help(num % 1000000);
} else {
ret = help(num / 1000000000) + " Billion " + help(num % 1000000000);
}
return ret.trim();
}
}