I delet element from vector in O(1) swap with last + pop_back.

As we can see the constraction of vector isn't broke like in other solutions.

```
class Solution {
public:
int removeElement(vector<int>& nums, int val) {
for (int i = 0; i < nums.size(); ++i) {
if (nums[i] == val) {
swap(nums[i], nums[nums.size() - 1]);
nums.pop_back();
--i;
}
}
return nums.size();
}
};
```