Find All Numbers Disappeared in an Array

Given an array of integers, 1 ≤ a[i] ≤ n (n = size of array), some elements appear twice and others appear once.

Find all the elements of [1, n] inclusive that do not appear in this array.

Could you do it without extra space and in O(n) runtime?

Example:

Input:

[4,3,2,7,8,2,3,1]

Output:

[5,6]

class Solution {
public:
    vector<int> findDisappearedNumbers(vector<int>& nums) {
        // put all numbers in correct positions 
        for (int i = 0; i < nums.size(); i++) {
            while (nums[i] != -1 && i != nums[i] - 1) {
                if (nums[i] == nums[nums[i] - 1]) {
                    nums[i] = -1;
                    break;
                }
                swap(nums[i], nums[nums[i] - 1]);
            }
        }
        
        // put all vacant numbers into the result array
        vector<int> result;
        for (int i = 0; i < nums.size(); i++) {
            if (nums[i] == -1)
                result.push_back(i + 1);
        }
        return result;
    }
};