<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:
  def countDigitOne(self, n: int) -> int:
    ans = 0

    pow10 = 1
    while pow10 <= n:
      divisor = pow10 * 10
      quotient = n // divisor
      remainder = n % divisor
      if quotient > 0:
        ans += quotient * pow10
      if remainder >= pow10:
        ans += min(remainder - pow10 + 1, pow10)
      pow10 *= 10

    return ans
16500