448. Find All Numbers Disappeared in an Array
     思路：Could you do it without extra space and in O(n) runtime? You may assume the returned list does not count as extra space. 不占用额外的空间并且时间复杂度在O(n)。把每个数减一，将其作为所谓的数组下标，出现的就取负，不出现的依旧是正数，如此找到缺失的整数。

class Solution(object):
    def findDisappearedNumbers(self, nums):
        """
        :type nums: List[int]
        :rtype: List[int]
        """
        for i in range(len(nums)):
            index = abs(nums[i])-1
            nums[index] = - abs(nums[index])
            
        return [i +1 for i in range(len(nums)) if nums[i]>0]

好枯燥，得换种学习方式了，半天也没啥长进。