[Leetcode] First Missing Positive

Posted 2020-08-04

41. 

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.

 

Solution:

Scan the array, for every index, put the number into the right place. Then scan the array for the 2nd time, find the wrong number should be easy.

Especially notice the loop condition.

class Solution(object):
    def firstMissingPositive(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """ 
        n = len(nums)
        for i in xrange(n):
            while n>=nums[i]>0 and nums[i] != nums[nums[i]-1]:    #especially notice the condition, num[i]!=nums[nums[i]-1], which means the current number is corret in the corresponding index
                tmp = nums[i]-1
                nums[i], nums[tmp] = nums[tmp], nums[i]
        for i in xrange(n):
            if nums[i] != i+1:
                return i+1
        return n+1