[Solved] 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.

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Question

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
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

Python Solution

class Solution:
    def firstMissingPositive(self, nums: List[int]) -> int:
        d = {}
        l = len(nums)
        if nums == []:
            return 1
        if nums==[1]:
            return 2
        
        for i in range(l):
            d[nums[i]] = i
        
        for i in range(1,l+1):
            if i not in d:
                if i==None:
                    break
                else:
                    return i
        return l+1

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