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From: YDZ <ydz627@gmail.com>
Date: Thu, 11 Jul 2019 22:40:09 +0800
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+# [268. Missing Number](https://leetcode.com/problems/missing-number/)
+
+
+## 题目:
+
+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?
+
+
+## 题目大意
+
+给定一个包含 0, 1, 2, ..., n 中 n 个数的序列，找出 0 .. n 中没有出现在序列中的那个数。算法应该具有线性时间复杂度。你能否仅使用额外常数空间来实现?
+
+
+
+## 解题思路
+
+
+- 要求找出 `0, 1, 2, ..., n` 中缺失的那个数。还是利用异或的性质，`X^X = 0`。这里我们需要构造一个 X，用数组下标就可以了。数字下标是从 `[0，n-1]`，数字是 `[0，n]`，依次把数组里面的数组进行异或，把结果和 n 再异或一次，中和掉出现的数字，剩下的那个数字就是之前没有出现过的，缺失的数字。