style(p4, p9): fix style issue

problems/p4/src/main.rs

@@ -13,12 +13,12 @@ fn main() -> Result<(), Box<dyn Error>> {
         // 对密文的每个字节应用异或操作，得到明文
         let plaintext: Vec<u8> = cipher_bytes.iter().map(|&byte| byte ^ key).collect();
         for i in 0..=(plaintext.iter().len() - window_size) {
-            if let Ok(text) = std::str::from_utf8(&plaintext[i..i + window_size]) {
-                if is_valid_english(text, None) {
+            if let Ok(text) = std::str::from_utf8(&plaintext[i..i + window_size])
+                && is_valid_english(text, None)
+            {
                 println!("Found valid sentence with key {key}: {text}");
             }
         }
-        }
         // 尝试将字节转换为字符串
     }
 

problems/p9/src/main.rs

@@ -15,6 +15,7 @@ fn pkcs7_padding(data: &mut Vec<u8>, block_size: usize) {
     data.extend(vec![padding_length as u8; padding_length]);
 }
 
+#[allow(dead_code)]
 fn pkcs7_unpadding(input: &[u8]) -> Result<Vec<u8>> {
     if input.is_empty() {
         return Err(anyhow!("Input cannot be empty"));