这道题跟 41. First Missing Positive 很像！
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?
link:{sourcedir}/_0268_MissingNumber.java[role=include]