For each number i in nums,

we mark the number that i points as negative.

Then we filter the list, get all the indexes

who points to a positive number.

Since those indexes are not visited.

```
class Solution(object):
def findDisappearedNumbers(self, nums):
"""
:type nums: List[int]
:rtype: List[int]
"""
# For each number i in nums,
# we mark the number that i points as negative.
# Then we filter the list, get all the indexes
# who points to a positive number
for i in xrange(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]
```