Try to put every integer into its position since a[i] < n with "pointer" chasing

```
def findDisappearedNumbers(self, nums):
for i, v in enumerate(nums):
while nums[v-1] != v:
nums[v-1], v = v, nums[v-1]
ans = [i+1 for i, v in enumerate(nums) if i+1 != v]
return ans
```