41-first-missing-positive

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Description

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

Example 2:

Input: nums = [3,4,-1,1]
Output: 2

Example 3:

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

 

Constraints:

• 1 <= nums.length <= 5 * 105
• -231 <= nums[i] <= 231 - 1

Solution(Python)

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