<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 segregateArr(vector<int>& arr) {
        int j = 0;
        int n = arr.size();
        for (int i = 0; i < n; i++) {
            if (arr[i] <= 0) {
                swap(arr[i], arr[j]);
                j++;
            }
        }
        return j;
    }

    int findMissingPositive(vector<int>& arr, int size) {
        for (int i = 0; i < size; i++) {
            int value = abs(arr[i]);
            if (value <= size && arr[value - 1] > 0) {
                arr[value - 1] = -arr[value - 1];
            }
        }

        for (int i = 0; i < size; i++) {
            if (arr[i] > 0) {
                return i + 1;
            }
        }
        return size + 1;
    }

    int firstMissingPositive(vector<int>& arr) {
        int n = arr.size();
        int shift = segregateArr(arr);
        vector<int> v;
        for (int i = shift; i < n; i++)
            v.push_back(arr[i]);
        return findMissingPositive(v, n - shift);
    }
};

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