< Back 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.