LeetCode #273 - Integer to English Words

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本文介绍了一种将非负整数转换为其英文单词表示的方法，适用于小于INT_MAX的所有输入。通过详细的代码示例，展示了如何处理从个位到十亿级别的数字，并提供了四个实例输出，包括One Hundred Twenty Three、Twelve Thousand Three Hundred Forty Five、One Million Two Hundred Thirty Four Thousand Five Hundred Sixty Seven和One Billion Two Hundred Thirty Four Million Five Hundred Sixty Seven Thousand Eight Hundred Ninety One。

题目描述：

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

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 {
public:
    string numberToWords(int num) {
        if(num == 0) return "Zero";
        return helper(num);        
    }
    
    string helper(int num)
    {
        string result;
        vector<string> num_list = {"One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine"};
        vector<string> tens_place = {"Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};
        vector<string> list = {"Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
        int one_billion = 1000000000;
        int one_million = 1000000;
        int one_thousand = 1000;
        if(num >= one_billion) 
            result = helper(num / one_billion) + " Billion " + helper(num % one_billion);
        else if(num >= one_million)
            result = helper(num / one_million) + " Million " + helper(num % one_million);
        else if(num >= one_thousand)
            result = helper(num / one_thousand) + " Thousand " + helper(num % one_thousand);
        else
        {
            int a = num / 100, b = num % 100, c = num % 10;
            if(a > 0) 
            {
                result += num_list[a - 1] + " Hundred";
                if(b > 0) result += " ";
            }
            if(b < 20 && b >= 10)  result += list[b - 10];
            else if(b > 0 && b < 10) result += num_list[c - 1];
            else if(b >= 20)
            {
                result += tens_place[b / 10 - 2];
                if(c > 0) result += " " + num_list[c - 1];
            }
        }
        while(result.back() == ' ') result.pop_back();
        return result;
    }
};