# 273. Integer to English Words

[Link](https://leetcode.com/problems/integer-to-english-words/description/)

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"
```

**Solution**

這題做法很巧妙，可以理解但應該無法想到，只能記起來了。

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

    public String numberToWords(int num) {
        if(num == 0) return "Zero";
        int i = 0;
        String ans = "";
        while(num != 0){
            if(num%1000 != 0){
                ans = helper(num%1000) + THOUSANDS[i] +  " "  + ans;
            }
            num = num/1000;
            i++;
        }
        return ans.trim();
    }
    
    public String helper(int num){
        if(num == 0) return "";
        else if (num < 20){
            return UNDER_TWENTY[num] + " ";
        }
        else if (num < 100){
            return TENS[num/10] + " " + helper(num%10);
        }
        else{
            return UNDER_TWENTY[num/100] + " Hundred " + helper(num%100);
        }
    }
}
```
