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

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> n = 13
<strong>Output:</strong> 6
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> n = 0
<strong>Output:</strong> 0
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>0 &lt;= n &lt;= 10<sup>9</sup></code></li>
</ul>

class Solution {
public:
    int countDigitOne(int n) {
        long long count = 0;
        long long factor = 1;
        while (factor <= n) {
            // Calculate the parts
            long long lowerNumbers = n - (n / factor) * factor;
            long long currentDigit = (n / factor) % 10;
            long long higherNumbers = n / (factor * 10);
            
            // Counting digit `1` in the current place
            if (currentDigit == 0) {
                count += higherNumbers * factor;
            } else if (currentDigit == 1) {
                count += higherNumbers * factor + lowerNumbers + 1;
            } else {
                count += (higherNumbers + 1) * factor;
            }
            
            // Move to the next digit place
            factor *= 10;
        }
        return count;
    }
};

7400