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268. Missing Number

Description

Given an array containing n distinct numbers taken from 0, 1, 2, ..., n, find the one that is missing from the array.

Example 1:

Input: [3,0,1]
Output: 2


Example 2:

Input: [9,6,4,2,3,5,7,0,1]
Output: 8


Note:
Your algorithm should run in linear runtime complexity. Could you implement it using only constant extra space complexity?