题目
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"

答案

简化过的答案

class Solution {
    String[] map = {"Twenty","Thirty","Forty","Fifty","Sixty","Seventy","Eighty","Ninety",
                    "Hundred","Thousand","Million","Billion"};
    String[] map_1_19 = {"", "One","Two","Three","Four","Five","Six","Seven","Eight",
                         "Nine","Ten","Eleven","Twelve","Thirteen","Fourteen",
                         "Fifteen","Sixteen","Seventeen","Eighteen","Nineteen"};

    public String numberToWords(int num) {
        if(num == 0) return "Zero";
        return recur(num);
    }

    public String recur(int num) {
        StringBuilder sb = new StringBuilder();
        String strnum = Integer.toString(num);
        int len = strnum.length(), prefix, prefix_len, divider, index;

        if(len == 10) {divider = 1000000000;index = 11;}
        else if(len < 10 && len >= 7) {divider = 1000000; index = 10;}
        else if(len < 7 && len >= 4) {divider = 1000; index = 9;}
        else if(len == 3){divider = 100; index = 8;}
        else if(num < 100 && num > 19){divider = 10; index = (num / divider) - 2;}
        else {return map_1_19[num];}

        prefix = num / divider;
        if(divider == 10) sb.append(map[index]);
        else sb.append(recur(prefix)).append(" ").append(map[index]);
        String ret = recur(num % divider);
        if(!ret.equals("")) sb.append(" ").append(ret);

        return sb.toString();
    }
}

未简化的答案(对每个case有很详细的记录)

class Solution {
String[] map = {"","","Hundred","Thousand","","","Million","","","Billion"};
    String[] map_tens = {"Ten","Eleven","Twelve","Thirteen","Fourteen","Fifteen","Sixteen","Seventeen","Eighteen","Nineteen"};
    String[] map_gt_tens = {"","Twenty","Thirty","Forty","Fifty","Sixty","Seventy","Eighty","Ninety"};
    String[] map_sm_tens = {"One","Two","Three","Four","Five","Six","Seven","Eight","Nine"};

    public String numberToWords(int num) {
        if(num == 0) return "Zero";
        return recur(num);
    }

    public String recur(int num) {
        StringBuilder sb = new StringBuilder();
        String strnum = Integer.toString(num);
        int len = strnum.length();
        int prefix, prefix_len;

        if(len == 10) {
            // Billion
            prefix = num / 1000000000;
            String prefix_repr = recur(prefix);
            sb.append(prefix_repr);
            sb.append(" ");
            sb.append(map[9]);

            String ret = recur(num % 1000000000);
            if(!ret.equals("")) {
                sb.append(" ");
                sb.append(ret);
            }
        }
        else if(len < 10 && len >= 7) {
            // Million
            prefix = num / 1000000;
            String prefix_repr = recur(prefix);
            sb.append(prefix_repr);
            sb.append(" ");
            sb.append(map[6]);

            String ret = recur(num % 1000000);
            if(!ret.equals("")) {
                sb.append(" ");
                sb.append(ret);
            }
        }
        else if(len < 7 && len >= 4) {
            // Thousand
            prefix = num / 1000;
            String prefix_repr = recur(prefix);
            sb.append(prefix_repr);
            sb.append(" ");
            sb.append(map[3]);
            String ret = recur(num % 1000);
            if(!ret.equals("")) {
                sb.append(" ");
                sb.append(ret);
            }
        }
        else if(len == 3) {
            // Hundred
            prefix = num / 100;
            String prefix_repr = recur(prefix);
            sb.append(prefix_repr);
            sb.append(" ");
            sb.append(map[2]);
            String ret = recur(num % 100);
            if(!ret.equals("")) {
                sb.append(" ");
                sb.append(ret);
            }

        }
        else if(len == 2) {
            prefix = num / 10;
            if(prefix == 1) {
                sb.append(map_tens[num - 10]);
            }
            else {
                sb.append(map_gt_tens[prefix - 1]);
                String ret = recur(num % 10);
                if(!ret.equals("")) {
                    sb.append(" ");
                    sb.append(ret);
                }
            }
        }
        else if(len == 1) {
            // Do nothing in case of 0, for exampty 30 = thirty + [nothing]
            if(num != 0) {
                sb.append(map_sm_tens[num - 1]);
            }
        }
        return sb.toString();
    }
}