diff --git a/day9/case3.cpp b/day9/case3.cpp
new file mode 100644
index 0000000..8b1f9c0
--- /dev/null
+++ b/day9/case3.cpp
@@ -0,0 +1,28 @@
+// 定义函数, 实现字符串的反转并输出
+// 例如: abcdefg -> gfedcba
+#include <iostream>
+
+using namespace std;
+
+void reverse(char str[], int len)
+{
+    int i = 0;           // 为什么要从 0 开始? 因为字符串的第一个字符是 str[0]
+    int j = len - 1 - 1; // 为什么要再减 1? 因为字符串最后一个字符是 '\0'
+    for (; i < j; i++, j--)
+    {
+        str[i] = str[i] ^ str[j];
+        str[j] = str[i] ^ str[j];
+        str[i] = str[i] ^ str[j];
+    }
+}
+
+int main()
+{
+    char str[] = "abcdefg";
+    int len = sizeof(str) / sizeof(str[0]);
+    cout << "原字符串: " << str << endl;
+    reverse(str, len);
+    cout << "反转后: " << str << endl;
+
+    return 0;
+}
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