問題描述：

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"

解題思路：

我們可以來看一下0-1 billion之間：

　　　　　　　　1　　0　　0　　0　　0　　0　　0　　0　　0　　0

　　　　　　　billion　　　　　　　 million　　　　　　thousand hundred　　　

　　　　　　　　 |-----------| |-----------| |------------|

觀察可以發現：每3個0更換一個單位。

我們可以設置一個unit 的hash map，存儲單位，註意0最好也存上相對應的“”

再對1-9， 10-19， 20， 30， 40， 50， 60， 70 ，80， 90進行存儲

對num進行模1000操作取出當前後3位，對後三位進行數文轉換後根據當前技術值count加後面的單位。

需要註意的是：

　　什麽時候插入空格！

代碼：

class Solution {
public:
    string numberToWords(int num) {
        unordered_map<int,string> dict ={
            {1, "
 
One"}, {2, "Two"}, {3, "Three"}, {4, "Four"}, {5, "Five"}, {6, "Six"}, {7, "Seven"}, {8, "Eight"}, {9, "Nine"},
            {10, "Ten"},{11, "Eleven"}, {12, "Twelve"}, {13, "Thirteen"}, {14, "Fourteen"}, {15, "Fifteen"},{16, "Sixteen"}, 
            {17, "Seventeen"}, {18, "Eighteen"}, {19, "Nineteen"}, {20, "Twenty"}, {30, "Thirty"}, {40, "Forty"}, {50, "Fifty"},
            {60, "Sixty"}, {70, "Seventy"}, {80, "Eighty"}, {90, "Ninety"}
        };
        unordered_map<int,string> unit = {{0, ""},{1, " Thousand"}, {2, " Million"}, {3," Billion"}};
        string ret = "";
        int count = 0;
        if(num == 0)
            return "Zero";
        while(num > 0){
            int cur = num % 1000;
            if(cur != 0){
                string wn = toWords(cur, dict);
                wn += unit[count];
                if(ret.size() != 0){
                    ret = " "+ret;
                }
                ret = wn + ret;
            }
            count++;
            num /= 1000;
        }
        return ret;
        
    }
private:
    string toWords(int n, unordered_map<int, string> &m){
        string ret;
        int h = n/100;
        if(h != 0)
            ret = m[h] + " Hundred";
        n %= 100;
        if(n == 0)
            return ret;
        if(m.count(n)){
            if(ret.size() != 0)
                ret += " ";
            ret += m[n];
        }else{
            int t = n/10;
            if(t != 0){
                if(ret.size() != 0)
                    ret += " ";
                ret += m[t*10];
            }
            int d = n%10;
            if(d != 0){
                if(ret.size() != 0)
                    ret += " ";
                ret += m[d];
            }
        }
        return ret;
    }
};

273. Integer to English Words