First Missing Positive

Problem Statement

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.

Leetcode link

Example 1:

Input: nums = [1,2,0]
Output: 3
Explanation: The numbers in the range [1,2] are all in the array.

Example 2:

Input: nums = [3,4,-1,1]
Output: 2
Explanation: 1 is in the array but 2 is missing.

Example 3:

Input: nums = [7,8,9,11,12]
Output: 1
Explanation: The smallest positive integer 1 is missing.

Constraints:

• 1 <= nums.length <= 10⁵
• -2³¹ <= nums[i] <= 2³¹ - 1

Code

Python Code

class Solution(object):
        def firstMissingPositive(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        for i in range(len(nums)):
            tmp, nums[i] = nums[i], None
            while tmp is not None and 0 <= tmp-1 < len(nums) and tmp != nums[tmp-1]:
                nums[tmp-1], tmp = tmp, nums[tmp-1]

        for i in range(len(nums)):
            if nums[i] is None:
                return i+1
        return len(nums) + 1