41\. First Missing Positive

Hard

Given an unsorted integer array `nums`, return the smallest missing positive integer.

You must implement an algorithm that runs in `O(n)` time and uses constant extra space.

**Example 1:**

**Input:** nums = [1,2,0]

**Output:** 3 

**Example 2:**

**Input:** nums = [3,4,-1,1]

**Output:** 2 

**Example 3:**

**Input:** nums = [7,8,9,11,12]

**Output:** 1 

**Constraints:**

*   <code>1 <= nums.length <= 5 * 10<sup>5</sup></code>
*   <code>-2<sup>31</sup> <= nums[i] <= 2<sup>31</sup> - 1</code>