老晨Leetcode 273. Integer to English Words 整数转单词

把一个非负整数转成英文单词的形式。给定的输入一定比2^31-1小。

实现不是特别难，但是细节多，网上大神的解法值得背诵
关键点一
有很多字符串常量的情况用String[] 很清楚，比switch或者map要好看很多
而且多注意第一个为0的Index是不是需要是“”，方便后边拿取数据
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”};

关键点二：
处理起来肯定还是先处理最后三位会方便很多，不信你试试从最前面开始，很不好处理（这也是处理int数据的常见技巧，先处理%10或者%1000最后几位）
这样的话跟最终的结果是反过来的，我的做法是把要输出的字符串反着存进List, 再用StringBuilder 反着拼起来
但大神这种做法是更漂亮的，用
words = helper(num % 1000) +THOUSANDS[i] + " " + words;
完美搞定
不一定非要用StringBuilder ，反过来字符串拼接更简洁易懂

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 words = "";
    
    while (num > 0) {
        if (num % 1000 != 0)
    	    words = helper(num % 1000) +THOUSANDS[i] + " " + words;
    	num /= 1000;
    	i++;
    }
    
    return words.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);
}

我的代码

class Solution {
    private static final String[] less20 = {"","One","Two","Three","Four","Five","Six","Seven","Eight","Nine","Ten","Eleven","Twelve","Thirteen","Fourteen","Fifteen","Sixteen","Seventeen","Eighteen","Nineteen"};
    private static final String[] tens = {"","Ten","Twenty","Thirty","Forty","Fifty","Sixty","Seventy","Eighty","Ninety"};
    private static final String[] bigs = {"Hundred","Thousand","Million", "Billion"};
    public String numberToWords(int num) {
        if(num == 0) return "Zero";
        List<String> list = new ArrayList();
        int big = 0;
        while(num != 0) {
            getCurString(list, big++, num % 1000);
            num = num/1000;
        }      
        return getString(list);
    }  
    private void getCurString(List<String> list, int bigIdx, int cur) {
        if(bigIdx != 0 && cur != 0) {
            list.add(bigs[bigIdx]);
        }
        int last2 = cur % 100;
        if(last2 < 20) {
            list.add(less20[last2]);
        } else {
            list.add(less20[last2 % 10]);
            list.add(tens[last2 / 10]);
        }
        int hundred = cur /100;
        if(hundred != 0) {
            list.add(bigs[0]);
            list.add(less20[hundred]);
        }
    }
    private String getString(List<String> list) {
        Collections.reverse(list);
        StringBuilder sb = new StringBuilder();
        for(String s: list) {
            if(s.length() == 0) continue;
            sb.append(s).append(" ");
        }
        sb.setLength(sb.length() -1);
        return sb.toString();
    }
}