Problem 1.py

#time complexity = O(n)
#space complexity = O(n)
class Solution(object):
    def findDisappearedNumbers(self, nums):
        """
        :type nums: List[int]
        :rtype: List[int]
        """

        # Hash table for keeping track of the numbers in the array
        # Note that we can also use a set here since we are not 
        # really concerned with the frequency of numbers.
        hash_table = {}

        # Add each of the numbers to the hash table
        for num in nums:
            hash_table[num] = 1

        # Response array that would contain the missing numbers
        result = []

        # Iterate over the numbers from 1 to N and add all those
        # that don't appear in the hash table. 
        for num in range(1, len(nums) + 1):
            if num not in hash_table:
                result.append(num)

        return result