448. Find All Numbers Disappeared in an Array | LeetCode

// Solution 1
class Solution {
public:
    vector<int> findDisappearedNumbers(vector<int>& nums) {
        unordered_set<int> s(nums.begin(), nums.end());
        vector<int> ans(nums.size()-s.size());
        for(int i = 1, j = 0; i <= size(nums); i++)  
            if(!s.count(i)) ans[j++] = i;
        return ans;
    }
};