leetcode-448-find-all-numbers-disappeared-in-an-array.swift

/**

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https://leetcode.com/problems/find-all-numbers-disappeared-in-an-array/


Given an array of integers where 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? You may assume the returned list does not count as extra space.

Example:

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

Output:
[5,6]

*/

/**
Big O Annotation
Time complexity O(n) where n is the amount of elements in nums.
Space complexity O(n) where n is the amount of elements in nums.

The follow up was to try no extra space and constant time.
I will update this solution as soon as I come up with a decent idea.
*/
func findDisappearedNumbers(_ nums: [Int]) -> [Int] {
        
    guard !nums.isEmpty else {
        return []
    }
    
    var resultArray: Set<Int> = []
    
    for index in 1...nums.count {
        resultArray.insert(index)
    }
    
    for element in nums {
        if resultArray.contains(element) {
            resultArray.remove(element)
        }
    }
    
    return Array(resultArray)
}