448. Find All Numbers Disappeared in an Array 🌟

• GOOGLE, MICROSOFT

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.

Brute force

• For each number in the range [1,n], check if it is in the array.
• If not then add it in the result array.
• TC: O(N^2)
• SC: O(1)

Sort + search

• Sort the array.
• Then we can binary search the missing numbers OR linearly compare 2 adjacent numbers.
• TC: O(NlogN)
• SC: O(1)

Hash set

• We can store the value of each number in the array in a hash set.
• Then we can iterate over the range [1,n] and check if the number is in the hash set.
• TC: O(N)
• SC: O(N)

Space-Optimized using In-place modification

• We mark the present element as negative.
• then we iterate from [1,n] and check if there is number > 0, if its present then we add it in the answer array.
• TC: O(N)
• SC: O(1)

Code

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