273. Integer to English Words

Convert a non-negative integer to its english words representation. Given input is guaranteed to be less than 231 - 1.

Example 1:

Input: 123
Output: "One Hundred Twenty Three"

Example 2:

Input: 12345
Output: "Twelve Thousand Three Hundred Forty Five"

Example 3:

Input: 1234567
Output: "One Million Two Hundred Thirty Four Thousand Five Hundred Sixty Seven"

Example 4:

Input: 1234567891
Output: "One Billion Two Hundred Thirty Four Million Five Hundred Sixty Seven Thousand Eight Hundred Ninety One"

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

你品，你细细品

上面的while循环时计算thousands的，包括thousand，million，billion，1000 1000算的，对应数组的位置

下面的helper计算小于1000，最复杂的就是ninety hundred ninety nine之类的，所以按20，100区分，对应数组位置，递归调用

class Solution {
  public String numberToWords(int num) {
        if(num==0) {
            return "Zero";
        }
        return helper(num);
    }
    public String helper(int num) {
        String[] words = new String[] {"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine",
        "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen", 
        "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};
        StringBuilder res = new StringBuilder();
        
        if(num>=1000000000) {
            res.append(helper(num/1000000000)).append(" Billion ").append(helper(num%1000000000));
        } else if(num>=1000000) {
            res.append(helper(num/1000000)).append(" Million ").append(helper(num%1000000));
        } else if(num>=1000) {
            res.append(helper(num/1000)).append(" Thousand ").append(helper(num%1000));
        } else if(num>=100) {
            res.append(helper(num/100)).append(" Hundred ").append(helper(num%100));
        } else if(num>=20) {
            res.append(words[(num-20)/10+20]).append(" ").append(words[num%10]);
        } else {
            res.append(words[num]);
        }
        
        return res.toString().trim();
    }
}