First Missing Positive

Given an unsorted integer array, find the smallest missing positive integer.

Example 1:

Input: [1,2,0]
Output: 3
Example 2:

Input: [3,4,-1,1]
Output: 2
Example 3:

Input: [7,8,9,11,12]
Output: 1
Note:

Your algorithm should run in O(n) time and uses constant extra space.



My code:
class Solution:
    def firstMissingPositive(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        tmp = set(nums)
        for i in range(len(tmp) + 1):
            if i + 1 not in tmp:
                return i + 1
                
Solution:
https://leetcode.com/problems/first-missing-positive/