0448-Find All Numbers Disappeared in an Array.md

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]

Solution

O(nums) Time, O(nums) Space:

class Solution {
    func findDisappearedNumbers(_ nums: [Int]) -> [Int] {
        guard !nums.isEmpty else { return nums }
        var set : Set<Int> = []
        for num in nums {
            set.insert(num)
        }
        var result : [Int] = []
        for num in 1...nums.count {
            if !set.contains(num) {
                result.append(num)
            }
        }
        return result
    }
}

O(nums) Time, O(1) Space:

class Solution {
    func findDisappearedNumbers(_ nums: [Int]) -> [Int] {
        var nums = nums, i = 0, result : [Int] = []
        while i < nums.count {
            if i != nums[i]-1 && nums[i] != nums[nums[i]-1] {
                nums.swapAt(i, nums[i]-1)
            } else {
                i+=1
            }
        }
        for (index, value) in nums.enumerated() {
            if index != value-1 {
                result.append(index+1)
            }
        }
        return result
    }
}