This problem is not difficult. But it is not easy to have a bug-free code. As you write your codes according to the hints, the most important thing is to handle all the edge cases without making the code ugly :-) Well, the key to simplify the code is to us hard coding. provides a nice solution, whose Java code is rewritten below in C++. Since C++ has no convenient functions like trim and I do not want to create one on my own, I handle the " " and ' ' carefully using some tricks from  (for example, I include the space in the words of the numbers).

1 class Solution { 2 public:  3     string numberToWords(int num) { 4         vector
     
       bigs = { "", " Thousand", " Million", " Billion" }; 5         int i = 0; 6         string ans; 7         while (num) { 8             if (num % 1000) ans = numberWords(num % 1000) + bigs[i] + ans; 9             i++, num /= 1000;10         }11         return ans.length() ? ans.substr(1) : "Zero";12     }13 private:14     string numberWords(int num) {15         char* one[] = { "", " One", " Two", " Three", " Four", " Five", " Six", " Seven", " Eight", " Nine" };16         char* ones[] = { " Ten", " Eleven", " Twelve", " Thirteen", " Fourteen", " Fifteen", " Sixteen", " Seventeen", " Eighteen", " Nineteen" };17         char* tens[] = { "", "", " Twenty", " Thirty", " Forty", " Fifty", " Sixty", " Seventy", " Eighty", " Ninety" };18         string ans;19         if (num > 99) ans += string(one[num / 100]) + " Hundred";20         num %= 100;21         if (num > 9 && num < 20) ans += ones[num % 10];22         else {23             if (num > 19) ans += tens[num / 10];24             num %= 10;25             if (num) ans += one[num];26         }27         return ans;28     }29 };