https://leetcode.com/problems/missing-number/
268. Missing Number
给一个包含[0, n]范围内不同数字的阵列nums，传回范围内唯一缺少的数字
Example 1:
Input: nums = [3,0,1]
Output: 2
Example 2:
Input: nums = [0,1]
Output: 2
Example 3:
Input: nums = [9,6,4,2,3,5,7,0,1]
Output: 8
Python3 code: