Use count sort to find disappeared numbers.

```
class Solution {
public:
vector<int> findDisappearedNumbers(vector<int>& nums) {
int i = 0;
while (i < nums.size()) {
if (nums[i] != nums[nums[i] - 1]) swap(nums[i], nums[nums[i] - 1]);
else ++i;
}
vector<int> resVec;
for (int i = 0; i < nums.size(); ++i) {
if (nums[i] != i + 1) resVec.push_back(i + 1);
}
return resVec;
}
};
```