<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:
    def firstMissingPositive(self, nums: List[int]) -> int:
        # # O(n) space: Create a set and iterate from 1 ->
        # for index, num in enumerate(nums):
        #     if nums[index] != val-1:
        #         temp = nums[index]
        #         nums[index] = 0
        #         place_in_pos(temp)

        # def place_in_pos(val):
        #     if val <= 0 or val > len(nums):
        #         return
        #     if nums[val] == val-1:
        #         return
            
        #     temp = nums[index]
        #     nums[index] = val
            
        #     place_in_pos(temp)
        def place_in_pos(val):
            if val <= 0 or val >= len(nums):
                return
            if nums[val] == val:
                return
            
            temp = nums[val]
            nums[val] = val
            place_in_pos(temp)
    
        nums = [0] + nums
        for index in range(1, len(nums)):
            val = nums[index] 
            if index != val:
                temp = nums[index]
                nums[index] = 0
                place_in_pos(temp)

        for i in range(1, len(nums)):
            if nums[i] == 0:
                return i
        
        return len(nums)

# [0,1,2,0]
#  0 1 2 3

# [0,1,2,0]
#  0 1 2 3


#  0 1 2
# [1,2,0]

# [3,4,_,1]

# [1,2,3,4]
#  1 2 3 4
# [1,0,3,4]

# [_, _, _, _, _, _]
#  0  1. 2  3. 4. 5

# out_of_bounds:
#  - <= 0
#  - > len(nums)

# [3,4,3,1]

# for val in nums:
#     temp = nums[val-1]
#     nums[val-1] = val

# [-1,3,3,3,3,3,3] 
#     1 2 3 4 5 6

# [1,0,3,4] 


35026