https://leetcode.com/problems/missing-number

This problem is a simple version of the problem 645. Set Mismatch, so if you solved it previously, this one will be easy. Here you can look at my solution:

https://leetcode.com/problems/set-mismatch/discuss/1089475/python-on-timeo1-space-math-solution-explained

The idea is the following: let us sum all numbers between 1 and n and then subtract sum of all nums and in the end we will have exaclty number we need.

Time complexity is O(n), space complexity is O(1).

class Solution:
    def missingNumber(self, nums):
        return len(nums)*(len(nums)+1)//2 - sum(nums)

If you like the solution, you can upvote it on leetcode discussion section: Problem 0268