Solution43.java

class Solution {
    public int countDigitOne(int n) {
    int count = 0;
    
    for (long digit = 1; digit <= n; digit *= 10) {
        long before = n / digit, after = n % digit;
        count += (before + 8) / 10 * digit;
        if (before % 10 == 1) {
            count += after + 1;
        }
    }
    
    return count;
  }

}