41. First Missing Positive

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.

把nums[index]交换换到nums[nums[index]-1]，再遍历

 

class Solution(object):
    def firstMissingPositive(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        r_num=1
        if nums==None or nums==[]:
            return r_num
        n=len(nums)    
        for index in range(n):
            while nums[index]>0 and nums[index]<=n and nums[index]!=nums[nums[index]-1]:
                temp=nums[index]
                nums[index]= nums[nums[index]-1]
                nums[temp - 1]=temp
        for index in range(n):
            if index+1!=nums[index]:
                return index+1
        return n+1