<p>Given an unsorted integer array <code>nums</code>. Return the <em>smallest positive integer</em> that is <em>not present</em> in <code>nums</code>.</p>

<p>You must implement an algorithm that runs in <code>O(n)</code> time and uses <code>O(1)</code> auxiliary space.</p>

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

<pre>
<strong>Input:</strong> nums = [1,2,0]
<strong>Output:</strong> 3
<strong>Explanation:</strong> The numbers in the range [1,2] are all in the array.
</pre>

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

<pre>
<strong>Input:</strong> nums = [3,4,-1,1]
<strong>Output:</strong> 2
<strong>Explanation:</strong> 1 is in the array but 2 is missing.
</pre>

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

<pre>
<strong>Input:</strong> nums = [7,8,9,11,12]
<strong>Output:</strong> 1
<strong>Explanation:</strong> The smallest positive integer 1 is missing.
</pre>

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

<ul>
	<li><code>1 &lt;= nums.length &lt;= 10<sup>5</sup></code></li>
	<li><code>-2<sup>31</sup> &lt;= nums[i] &lt;= 2<sup>31</sup> - 1</code></li>
</ul>

// class Solution {
// public:
//     int firstMissingPositive(vector<int>& nums) {
//         bool contains_one = false;
//         for(int x: nums){
//             if(x==1){
//                 contains_one = true;
//                 break;
//             }
//         }
//         if(!contains_one) return 1;
//         int n = nums.size();
//         if(n==1) return 2;
//         for(int i=0;i<n;i++){
//             if(nums[i]<=0 || nums[i]>n) nums[i]=1;
//         }
//         for(int i=0;i<n;i++){
//             int x=abs(nums[i]);
//             if(nums[x-1]>0) nums[x-1] *= -1;
//         }
//         for(int i=0;i<n;i++){
//             if(nums[i]>0) return i+1;
//         }
//         return n+1;
//     }
// };

class Solution {
public:
    int firstMissingPositive(vector<int>& nums) {
        nums.erase(remove_if(nums.begin(), nums.end(), [](int n) { return n <= 0; }), nums.end());

        sort(nums.begin(), nums.end());

        int target = 1;
        for (int n : nums) {
            if (n == target) {
                target++;
            } else if (n > target) {
                return target;
            }
        }
        
        return target;        
    }
};
22636