https://leetcode.com/problems/integer-to-english-words

integer-to-english-words.java

class Solution {
    String[] ones = {"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine"};
    String[] teens = {"Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", 
                          "Sixteen", "Seventeen", "Eighteen", "Nineteen"}; 
    String[] tens = {"", "Ten", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", 
                         "Seventy", "Eighty", "Ninety"};
    
    String[] ks = {"", "Thousand", "Million", "Billion"};
    public String numberToWords(int num) {
        if (num == 0) {
            return "Zero";
        }
        StringBuilder sb = new StringBuilder();
        String[] tokens = new String[4];
        int idx = 3;
        while(num > 0  && idx >= 0) {
            tokens[idx--] = parseHelper(num % 1000);
            num = num / 1000;
        }
        
        for (int i = 0; i < 4; i++) {
            if (tokens[i] != null && tokens[i].length() > 0) {
                if (sb.length() > 0) {
                    sb.append(" ");
                }
                sb.append(tokens[i]).append(" ").append(ks[3 - i]);
            }
        }
        return sb.toString().trim();
    }
  
    // most basic problem is a num with 3 digits
    private String parseHelper(int num) {
        // num -> [0, 999]
        if (num == 0) {
            return "";
        }
        StringBuilder sb = new StringBuilder();
        int[] digits = new int[3];
        int idx = 2;
        while(num > 0) {
            int token = num % 10;
            digits[idx--] = token;
            num = num / 10;
        }
        if (digits[0] > 0) {
            sb.append(ones[digits[0]]).append(" ").append("Hundred");
        }
        if (digits[1] >= 2) {
            if (sb.length() > 0) {
                sb.append(" ");
            }
            sb.append(tens[digits[1]]);
        }
        
        if (digits[1] == 1) {
            if (sb.length() > 0) {
                sb.append(" ");
            }
            sb.append(teens[digits[2]]);
            // no need to add ones
        }
        
        if (digits[1] != 1 && digits[2] > 0){
            if (sb.length() > 0) {
                sb.append(" ");
            }
            sb.append(ones[digits[2]]);
        }
        return sb.toString();
        
    }
    
}