LeetCode #41: First Missing Positive

最新推荐文章于 2025-08-11 15:06:51 发布

junchen1992

最新推荐文章于 2025-08-11 15:06:51 发布

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分类专栏：算法 LeetCode 文章标签： leetcode

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本文介绍了一种在未排序整数数组中寻找第一个缺失正整数的算法，该算法能在O(n)时间内运行并使用常量级空间。通过遍历数组并将元素放置到其在排序数组中的正确位置上，随后进行第二次扫描来找到缺失的正整数。

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Problem Statement

(Source) Given an unsorted integer array, find the first missing positive integer.

For example,
Given [1,2,0] return 3,
and [3,4,-1,1] return 2.

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

Analysis

The idea is to traverse through the whole array and put elements into their correct position in corresponding sorted array. Then another scan would find the first missing positive integer.

Solution

class Solution(object):
    def firstMissingPositive(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        if not nums:
            return 1

        n = len(nums)
        for index in xrange(n):
            num = nums[index]
            while num != index + 1:
                if 1 <= num <= n and nums[num - 1] != num:
                    nums[num - 1], nums[index] = nums[index], nums[num - 1]
                    num = nums[index]
                else:
                    break

        for index in xrange(n):
            if nums[index] != index + 1:
                return index + 1

        return nums[-1] + 1