[leetcode]273. Integer to English Words 整数转英文单词

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"

题目：

给定一个整数，像check一样将该数字用英文读出来

思路：

1. Coz given input is less than 2³¹ - 1, we can say that the highest digit level is billion 

2. We divide input into N parts, all the parts follow the similar partten of converting.  We use helper function to help recursive call.

代码：

class Solution {
    // coz array index begins at 0, so we use "" to empty a space
    private final String[] belowTen = new String[] {"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine"};
    private final String[] belowTwenty = new String[] {"Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
    private final String[] belowHundred = new String[] {"", "Ten", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};
    
    public String numberToWords(int num) {
        if (num == 0) return "Zero";
        return helper(num); 
    }
    
    private String helper(int num) {
        String result = new String();
        if (num < 10) result = belowTen[num];
        else if (num < 20) result = belowTwenty[num -10];
        // if num is smaller than 100, keep tens digit and add units digit
        else if (num < 100) result = belowHundred[num/10] + " " + helper(num % 10);
        // if num is smaller than thousand, call helper to get how many hundreds, call the helper to get the rest digits 
        else if (num < 1000) result = helper(num/100) + " Hundred " +  helper(num % 100);
        // if num is smaller than million, call helper to get how many thousands, call the helper to get the rest digits 
        else if (num < 1000000) result = helper(num/1000) + " Thousand " +  helper(num % 1000);
        // if num is smaller than billion, call helper to get how many millions, call the helper to get the rest digits 
        else if (num < 1000000000) result = helper(num/1000000) + " Million " +  helper(num % 1000000);
        else result = helper(num/1000000000) + " Billion " + helper(num % 1000000000);
        return result.trim();
    }
}