448. Find All Numbers Disappeared in an Array

Given an array nums of n integers where nums[i] is in the range [1, n], return an array of all the integers in the range [1, n] that do not appear in nums.

Follow up: Could you do it without extra space and in O(n) runtime? You may assume the returned list does not count as extra space.

class Solution {
public:
    vector<int> findDisappearedNumbers(vector<int>& nums) {
        vector<int> res;
        for(int i = 0; i < nums.size(); i++) {
            int idx = abs(nums[i]) - 1;
            nums[idx] = -(abs(nums[idx]));
        }
        for(int i = 0; i < nums.size(); i++) {
            if(nums[i] > 0) res.push_back(i+1);
        }
        return res;
    }
};