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268. Missing Number

Honestly, an annoying question. The optimal answer requires you to recall the formula for the sum of
an arithmetic sequence. The formula is as follows:

  sum = (n * (n + 1)) // 2

where n is the number of elements in the sequence. We can use this formula to find the sum of the
sequence of numbers from 0 to n. Then, we can subtract the sum of the given array from the sum of
the sequence of numbers from 0 to n. The result will be the missing number.

The solution is as follows:


  class Solution:
      def missingNumber(self, nums: List[int]) -> int:
          return ((len(nums) * (len(nums) + 1)) // 2) - sum(nums)


_ Time Complexity:

  O(n) - We calculate the length of the nums array. We also calculate the sum of the nums array.

_ Space Complexity:

  O(1) - We only use a constant amount of space to store the sum of the nums array and the length of
  the nums array.