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

先贴上我自己写的代码

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) {
            String result = new String();
            if(num == 0){
                result = "Zero";
 return result;
            }
            List<Integer> list = new ArrayList<Integer>();
            while(num != 0){
                list.add(num%10);
                num = num/10;
            }
            int length = list.size(),count = 0,k = 0;
            while(count < length){
                if(length - count >= 3){
                    String tmp  = count3Num(list, count, count+2);
                    if(length - count > 3 && tmp.length() == 0)
                        result = tmp + result;
                    else if(length - count == 3 || tmp.length() == 0)
                        result = tmp + THOUSANDS[k]+ result;
                    else
                        result = " " + tmp + THOUSANDS[k]+ result;
                    count += 3;
                    k++;
                }else{
                    result = count3Num(list, count, length-1) + THOUSANDS[k] + result;
                    count = length;
                }
            }
 return result;

        }

        public String count3Num(List<Integer> list,int s,int e){
            String result = new String();
            if(e-s == 2){
                if(list.get(e) != 0){
                    result = LESS_THAN_20[list.get(e)] + " " + "Hundred";
                    if(list.get(e-1) >= 2){
                        if(list.get(s) != 0){
                            result = result + " " + TENS[list.get(e-1)] + " " + LESS_THAN_20[list.get(s)];
                        }else{
                            result = result + " " + TENS[list.get(e-1)];
                        }
                    }
                    else{
                        if(list.get(s) == 0 && list.get(e-1) == 0){

                        }else{
                            result = result + " " + LESS_THAN_20[list.get(e-1)*10 + list.get(s)];
                        }
                    }
                }else{
                    if(list.get(e-1) >= 2){
                        if(list.get(s) != 0){
                            result =  TENS[list.get(e-1)] + " " + LESS_THAN_20[list.get(s)];
                        }else{
                            result = TENS[list.get(e-1)];
                        }
                    }
                    else{
                        if(list.get(s) == 0 && list.get(e-1) == 0){

                        }else{
                            result = LESS_THAN_20[list.get(e-1)*10 + list.get(s)];
                        }
                    }
                }

            }else if(e-s == 1){
                if(list.get(e) >= 2){
                    if(list.get(s) != 0){
                        result = TENS[list.get(e)] + " " + LESS_THAN_20[list.get(s)];
                    }else{
                        result = TENS[list.get(e)];
                    }
                }
                else
                    result = LESS_THAN_20[list.get(e)*10 + list.get(s)];
            }else{
                result = LESS_THAN_20[list.get(s)];
            }
 return result;
        }
}

这个应该算是最笨的方法来遍历所有的数字，大概的思路是把数从后往前三位一组进行读取，然后拼接到一起。这个方法需要考虑的情况很多。

接下来，需要改进

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";
        int i = 0;
        String result = new String();
        while(num != 0){
            if(num%1000 != 0){
                result = helper(num%1000) + THOUSANDS[i] + " " + result;
            }
            num /= 1000;
            i++;
        }
        return result.trim();
    }

    public 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);
    }


}

利用递归的方法来写，顿时就清爽了很多。