#ctci

NumToEnglish.java

public class NumToEnglish{
    // a number like 1,897,234 can be considered as
    // 1 million 897 thousand 234
    // we can divide them into several 3 digits sequence and insert
    // thousand or million in between
    // private String[] digits = {"One", ..., "Nine"};
    // private String[] tens = {"Ten", ..., "Ninety"};
    // private String[] teens = {"Eleven", ..., "Nineteen"};
    // private String[] bigs = {"", "Thousand", "Million"};
    
    public String convertNum(int num){
        if(num == 0) return "Zero";
        if(num < 0) return "Negative " + convertNum(-1 * num);
        
        int count = 0; // how many 3 digits we have
        String str = "";
        while(num > 0){
            str = convertNum100(num % 1000) + bigs[count] + " " + str;
            count++;
            num = num / 1000;
        }
        return str;
    }
    
    private String convertNum100(int num){
        String str = "";
        
        if(num >= 100){
            str = digits[num / 100 - 1] + " Hundred ";
            num = num % 100;
        }
        
        if(num >= 11 && num <= 19){
            str += teens[num - 11] + " ";
        }else if(num == 10 || num >= 20){
            str += tens[num / 10 - 1] + " ";
            num = num % 10;
        }
        if(num > 0) str += digits[num - 1] + " ";
        return str;
    }
}