add swap sample

data/1.Java初阶/5.Java中的变量与常量/1.变量的定义/swap.json

+{
+    "type": "code_options",
+    "author": "刘鑫",
+    "source": "swap.md",
+    "exercise_id":"accd5c6fca914d69990826e6123de37e"
+}
\ No newline at end of file

data/1.Java初阶/5.Java中的变量与常量/1.变量的定义/swap.md

+# 变量和赋值
+
+假设有a b两个整形变量，交换他们的值，可以用下列代码中的：
+
+## aop
+
+### before
+
+```java
+int a = 10;
+int b = 20;
+System.out.printf("before swap: a=%d, b=%d\n", a, b);
+```
+
+### after
+
+```java
+System.out.printf("after swap: a=%d, b=%d\n", a, b);
+```
+
+## 答案
+
+```java
+
+int tmp = a;
+a = b;
+b = tmp;
+
+```
+
+## 选项
+
+### 常量使用错误
+
+```java
+const int tmp = a;
+a = b;
+b = tmp;
+```
+
+### 类型错误
+
+```java
+auto tmp = a;
+a = b;
+b = tmp;
+```
+
+### Java 没有指针
+
+```java
+int* tmp = &a;
+a = b;
+b = tmp;
+```
+
+### 变量未定义
+
+```java
+tmp = a;
+a = b;
+b = tmp;
+```