<p>Given an integer <code>n</code>, return <em>the number of prime numbers that are strictly less than</em> <code>n</code>.</p>

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

<pre>
<strong>Input:</strong> n = 10
<strong>Output:</strong> 4
<strong>Explanation:</strong> There are 4 prime numbers less than 10, they are 2, 3, 5, 7.
</pre>

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

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

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> n = 1
<strong>Output:</strong> 0
</pre>

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

<ul>
	<li><code>0 &lt;= n &lt;= 5 * 10<sup>6</sup></code></li>
</ul>

class Solution {
public:
    
    int countPrimes(int n) {
        if(n < 2){
            return 0;
        }
        if(n < 3){
            return 0;
        }
        int odd = 0;
        int count = 0;
        vector<int> result((n-1)/2, 0);
        for(int i = 0; i < result.size(); i++){
            if(result[i] == 0){
                count++;
                odd = ((i+1)*2)+1;
                for(int j = i+odd; j < result.size(); j+=odd){
                    result[j] = 1;
                }
            }
        }
        if(((result.size()*2)+1 == n) && (result.back() == 0)){
            return count;
        }
        return count+1;
    }
};
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