Paste_Image.png

My code:

public class Solution {
    public String numberToWords(int num) {
        if (num < 0)
            return null;
        else if (num == 0)
            return "Zero";
        
        int billion = 1000000000;
        int million = 1000000;
        int thousand = 1000;
        String result = "";
        int tempBillion = num / billion;
        if (tempBillion != 0)
            result += getStr(tempBillion) + "Billion ";
        num = num - tempBillion * billion;
        int tempMillion = num / million;
        if (tempMillion != 0)
            result += getStr(tempMillion) + "Million ";
        num = num - tempMillion * million;
        int tempThousand = num / thousand;
        if (tempThousand != 0)
            result += getStr(tempThousand) + "Thousand ";
        num = num - tempThousand * thousand;
        if (num != 0)
            result += getStr(num);
        if (result.charAt(result.length() - 1) == ' ')
            return result.substring(0, result.length() - 1);
        else
            return result;
    }
    
    private String getStr(int num) {
        if (num <= 0)
            return "";
        String str = "";
        int hundred = num / 100;
        if (hundred != 0) {
            String hundredStr = getDigit(hundred);
            str += hundredStr + "Hundred ";
        }
        num = num - 100 * hundred;
        int ten = num / 10;
        if (ten != 0) {
            if (ten == 1) {
                str += getTen(num);
                return str;
            }
            else
                str += getMoreTen(ten);
        }
        num = num - 10 * ten;
        int digit = num % 10;
        if (digit != 0)
            str += getDigit(digit);
        
        return str;
    }
    
    private String getDigit(int digit) {
        if (digit <= 0)
            return "";
        switch (digit) {
            case 1:
                return "One ";
            case 2:
                return "Two ";
            case 3:
                return "Three ";
            case 4:
                return "Four ";
            case 5:
                return "Five ";
            case 6:
                return "Six ";
            case 7:
                return "Seven ";
            case 8:
                return "Eight ";
            case 9:
                return "Nine ";
            default:
                return "";
        }
    }
    
    private String getMoreTen(int ten) {
        if (ten <= 0)
            return "";
        switch (ten) {
            case 1:
                return "Ten ";
            case 2:
                return "Twenty ";
            case 3:
                return "Thirty ";
            case 4:
                return "Forty ";
            case 5:
                return "Fifty ";
            case 6:
                return "Sixty ";
            case 7:
                return "Seventy ";
            case 8:
                return "Eighty ";
            case 9:
                return "Ninety ";
            default:
                return "";
        }
    }
    
    private String getTen(int ten) {
        if (ten <= 0)
            return "";
        switch (ten) {
            case 10:
                return "Ten ";
            case 11:
                return "Eleven ";
            case 12:
                return "Twelve ";
            case 13:
                return "Thirteen ";
            case 14:
                return "Fourteen ";
            case 15:
                return "Fifteen ";
            case 16:
                return "Sixteen ";
            case 17:
                return "Seventeen ";
            case 18:
                return "Eighteen ";
            case 19:
                return "Nineteen ";
            default:
                return "";
        }
    }
    
    public static void main(String[] args) {
        Solution test = new Solution();
        System.out.println(test.numberToWords(10000));
    }
}

My test result:

Paste_Image.png

这道题目不难，但是比较烦。记住，一定要写子函数来帮助，否则，整个函数会显得无比的复杂。

日了狗了，为了完成每天三道题的量，学校作业来不及做了。不能这样！！！
**
总结： String, Pattern recognition, Math
**

Anyway, Good luck, Richardo!

My code:

public class Solution {
    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"};
    private final String[] THOUSANDS = {"", "Thousand", "Million", "Billion"};
    
    
    public String numberToWords(int num) {
        if (num == 0) {
            return "Zero";
        }
        String ret = "";
        int i = 0;
        while (num > 0) {
            if (num % 1000 != 0) {
                ret = helper(num % 1000) + THOUSANDS[i] + " " + ret;
            }
            num = num / 1000;
            i++;
        }
        
        return ret.trim();
    }
    
    private String helper(int num) {
        if (num == 0) {
            return "";
        }
        else if (num < 20) {
            return LESS_THAN_20[num] + " ";
        }
        else if (num < 100) {
            return TENS[num / 10] + " " + helper(num % 10);
        }
        else {
            return LESS_THAN_20[num / 100] + " Hundred " + helper(num % 100);
        }
    }
    
    public static void main(String[] args) {
        Solution test = new Solution();
        String ret = test.numberToWords(50868);
        System.out.println(ret);
    }
}

reference：
https://discuss.leetcode.com/topic/23054/my-clean-java-solution-very-easy-to-understand

看的网上的做法，很简洁。

分成了三种状态。
0-19

20,30,40,...90

thousand, million, billion

所以，给一个三位数，如何转换成英语，这个是核心功能。
于是就用几个数组。

注意的地方是：

else if (num < 100) {
     return TENS[num / 10] + " " + helper(num % 10);
 }

必须得用 helper(num % 10)
为什么不能直接 LESS_THAN_20[num % 10] + " "
因为如果是0，不用加空格。你这里无法判断。于是交给helper

Anyway, Good luck, Richardo! -- 09/23/2016