Integer to English Words

Convert a non-negative integer to its english words representation. Given input is guaranteed to be less than 231 - 1.

For example,

123 -> "One Hundred Twenty Three"
12345 -> "Twelve Thousand Three Hundred Forty Five"
1234567 -> "One Million Two Hundred Thirty Four Thousand Five Hundred Sixty Seven"

Hint:

1. Did you see a pattern in dividing the number into chunk of words? For example, 123 and 123000.
2. Group the number by thousands (3 digits). You can write a helper function that takes a number less than 1000 and convert just that chunk to words.
3. There are many edge cases. What are some good test cases? Does your code work with input such as 0? Or 1000010? (middle chunk is zero and should not be printed out)

Tips:

和hint说的差不多，三位一数，分好几种情况。

The run time complexity is O(1).

In the worst case scenario, when a number is greater than one billion, for example, the function helper is called a constant number of times. If you derive the recursion tree of this function, you'll see that the longest path in this situation (number greater than one billion) is: helper(num > 10^9) is the first to be called, then helper(10^6 < num < 10^9) is called (from the else statement), then helper(10^3 < num < 10^6), then helper(10^2 < num < 10^3), then helper(20 < num < 10^2), and finally, helper(num < 20), when returns a string.

Code:

public class Solution {
    private final String[] belowTen = new String[] {"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine"};
    private final String[] belowTwenty = new String[] {"Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
    private final String[] belowHundred = new String[] {"", "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) {
        String result = new String();
        if (num < 10) result = belowTen[num];
        else if (num < 20) result = belowTwenty[num -10];
        else if (num < 100) result = belowHundred[num/10] + " " + helper(num % 10);
        else if (num < 1000) result = helper(num/100) + " Hundred " +  helper(num % 100);
        else if (num < 1000000) result = helper(num/1000) + " Thousand " +  helper(num % 1000);
        else if (num < 1000000000) result = helper(num/1000000) + " Million " +  helper(num % 1000000);
        else result = helper(num/1000000000) + " Billion " + helper(num % 1000000000);
        return result.trim();
    }
}