Leecode brush questions record sword finger offer 43 The number of occurrences of 1 in integers 1 to n

topic

Enter an integer n , seek 1～n this n In the decimal representation of an integer 1 Number of occurrences .

for example , Input 12,1～12 These integers contain 1 The number of is 1、10、11 and 12,1 All in all 5 Time .

Example 1：

Input ：n = 12
Output ：5
Example 2：

Input ：n = 13
Output ：6

Limit ：

1 <= n < 2^31

answer

class Solution {
    
  public static int countDigitOne(int n) {
    
            int digit = 1, res = 0;
            int high = n / 10, cur = n % 10, low = 0;
            while(high != 0 || cur != 0) {
    
                if(cur == 0) res += high * digit;
                else if(cur == 1) res += high * digit + low + 1;
                else res += (high + 1) * digit;
                low += cur * digit;
                cur = high % 10;
                high /= 10;
                digit *= 10;
            }
            return res;
        }
}