feat: add python and java solutions to leetcode problem: No.0268

Author: yanglbme

README.md

@@ -117,6 +117,7 @@
 - [二进制中 1 的个数](/lcof/面试题15.%20二进制中1的个数/README.md)
 - [计数质数](/solution/0200-0299/0204.Count%20Primes/README.md)
 - [不用加减乘除做加法](/lcof/面试题65.%20不用加减乘除做加法/README.md)
+- [丢失的数字](/solution/0200-0299/0268.Missing%20Number/README.md)
 
 ### 栈和队列
 

README_EN.md

@@ -107,6 +107,7 @@ Complete solutions to [LeetCode](https://leetcode-cn.com/problemset/all/), [LCOF
 
 - [Set Mismatch](/solution/0600-0699/0645.Set%20Mismatch/README_EN.md)
 - [Count Primes](/solution/0200-0299/0204.Count%20Primes/README_EN.md)
+- [Missing Number](/solution/0200-0299/0268.Missing%20Number/README_EN.md)
 
 ### Stack & Queue
 

solution/0200-0299/0268.Missing Number/README.md

@@ -26,22 +26,55 @@
 
 <!-- 这里可写通用的实现逻辑 -->
 
+异或求解。两个相同的数异或的结果为 0。
+
+也可以用数学求解。求出 `[0..n]` 的和，减去数组中所有数的和，就得到了缺失的数字。
+
 <!-- tabs:start -->
 
 ### **Python3**
 
 <!-- 这里可写当前语言的特殊实现逻辑 -->
 
 ```python
-
+class Solution:
+    def missingNumber(self, nums: List[int]) -> int:
+        res = len(nums)
+        for i, v in enumerate(nums):
+            res ^= (i ^ v)
+        return res
 ```
 
 ### **Java**
 
 <!-- 这里可写当前语言的特殊实现逻辑 -->
 
+- 异或
+
 ```java
+class Solution {
+    public int missingNumber(int[] nums) {
+        int res = nums.length;
+        for (int i = 0, n = res; i < n; ++i) {
+            res ^= (i ^ nums[i]);
+        }
+        return res;
+    }
+}
+```
+
+- 数学
 
+```java
+class Solution {
+    public int missingNumber(int[] nums) {
+        int res = nums.length;
+        for (int i = 0, n = res; i < n; ++i) {
+            res += (i - nums[i]);
+        }
+        return res;
+    }
+}
 ```
 
 ### **...**

solution/0200-0299/0268.Missing Number/README_EN.md

@@ -37,13 +37,42 @@ Your algorithm should run in linear runtime complexity. Could you implement it u
 ### **Python3**
 
 ```python
-
+class Solution:
+    def missingNumber(self, nums: List[int]) -> int:
+        res = len(nums)
+        for i, v in enumerate(nums):
+            res ^= (i ^ v)
+        return res
 ```
 
 ### **Java**
 
+- XOR
+
 ```java
+class Solution {
+    public int missingNumber(int[] nums) {
+        int res = nums.length;
+        for (int i = 0, n = res; i < n; ++i) {
+            res ^= (i ^ nums[i]);
+        }
+        return res;
+    }
+}
+```
 
+- Math
+
+```java
+class Solution {
+    public int missingNumber(int[] nums) {
+        int res = nums.length;
+        for (int i = 0, n = res; i < n; ++i) {
+            res += (i - nums[i]);
+        }
+        return res;
+    }
+}
 ```
 
 ### **...**

solution/0200-0299/0268.Missing Number/Solution.java

@@ -1,8 +1,8 @@
 class Solution {
     public int missingNumber(int[] nums) {
         int res = nums.length;
-        for(int i = 0; i < nums.length; i++){
-            res += i - nums[i];
+        for (int i = 0, n = res; i < n; ++i) {
+            res ^= (i ^ nums[i]);
         }
         return res;
     }