source-code/Find_All_Numbers_Disappeared_in_an_Array.cpp

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