233 Number of Digit One

Given an integer n, count the total number of digit 1 appearing in all non-negative integers less than or equal to n.

For example:
Given n = 13,
Return 6, because digit 1 occurred in the following numbers: 1, 10, 11, 12, 13.

Hint:

1. Beware of overflow.

    

   class Solution {
   public:
       int countDigitOne(int n) {
           if (n <= 0)
               return 0;
   
           long long count = 0;    //It may cause integer overflow
   
           long long factor = 1;
   
           while (n / factor) {
               int lowerNum = n - (n / factor) * factor;
               int currentDigit = (n / factor) % 10;
               int higherDigit = n / (factor * 10);
   
               switch(currentDigit) {
                   case 0:
                       count += higherDigit * factor;
                       break;
                   case 1:
                       count += higherDigit * factor + (lowerNum + 1);
                       break;
                   default:
                       count += (higherDigit + 1) * factor;
               }
   
               factor *= 10;
           }
   
           return count;
       }
   };