<p>Given an array <code>nums</code> of <code>n</code> integers where <code>nums[i]</code> is in the range <code>[1, n]</code>, return <em>an array of all the integers in the range</em> <code>[1, n]</code> <em>that do not appear in</em> <code>nums</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<pre><strong>Input:</strong> nums = [4,3,2,7,8,2,3,1]
<strong>Output:</strong> [5,6]
</pre><p><strong class="example">Example 2:</strong></p>
<pre><strong>Input:</strong> nums = [1,1]
<strong>Output:</strong> [2]
</pre>
<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>n == nums.length</code></li>
	<li><code>1 &lt;= n &lt;= 10<sup>5</sup></code></li>
	<li><code>1 &lt;= nums[i] &lt;= n</code></li>
</ul>

<p>&nbsp;</p>
<p><strong>Follow up:</strong> Could you do it without extra space and in <code>O(n)</code> runtime? You may assume the returned list does not count as extra space.</p>

using namespace std;

class Solution {
public:
    vector<int> findDisappearedNumbers(vector<int>& nums)
    {
    sort(nums.begin(), nums.end());
    
    vector<int> unac;
    int n=nums.size();

/*     auto it  = unique(nums.begin(), nums.end()); 
  
    // Use erase method  to remove all the duplicates 
    // from the vector 
    nums.erase(it, nums.end());  */


        for(int b=1; b<=n; b++)
        {
       // cout<<nums[b-1]<<" ";

        if(b==nums[b-1])
        {}
        else                
        {unac.push_back((b)); 
        cout<<b<<" ";
        }
        }

       vector<int> returnThis;

    

    set_difference(unac.begin(), unac.end(), nums.begin(), nums.end(), back_inserter(returnThis));

    return returnThis;
        
      
    }
};
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