<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) {
        int ans = 0;
        long long p = 1;
        while(p <= n)
        {
            int a = n / p;
            int b = n % p;
            int d = a % 10;
            if(d == 0)
                ans += (a / 10) * p;
            else if(d == 1)
                ans += (a / 10) * p + b + 1;
            else
                ans += (a / 10 + 1) * p;
            p *= 10;
        }
        return ans;
    }
};
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