233. Number of Digit One

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LeetCode

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Problem

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

Example 1:

Input: n = 13
Output: 6

Example 2:

Input: n = 0
Output: 0

Code

class Solution {
    public int countDigitOne(int n) {
        int res = 0;
        for (long k = 1; k <= n; k *= 10) {
            long r = n / k;
            long m = n % k;
            
            res += (r + 8) / 10 * k + (r % 10 == 1 ? m + 1 : 0);
        }
        
        return (int)res;
    }
}