human
I found an interesting problem on https://leetcode.com/problems/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.

I tried it in Python, but could not do it. Can you solve it?

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What is a Python program to find the smallest missing positive integer?
assistant
#Here is Python program to find the smallest missing positive integer.

class Solution:
     def firstMissingPositive(self, nums):
         """
         :type nums: List[int]
         :rtype: int
         """
         n = len(nums)
         for i in range(n):
             while nums[i] > 0 and nums[i] <= n and nums[i] != i+1 and nums[i] != nums[nums[i]-1]:
                 nums[nums[i] - 1], nums[i] = nums[i], nums[nums[i]-1]
         for i in range(n):
             if nums[i] != i + 1:
                 return i + 1
         return n + 1