Several bug fixes.

codes/cpp/chapter_hashing/array_hash_map.cpp

@@ -47,7 +47,7 @@ class ArrayHashMap {
         int index = hashFunc(key);
         Pair *pair = buckets[index];
         if (pair == nullptr)
-            return nullptr;
+            return "";
         return pair->val;
     }
 

codes/cpp/chapter_hashing/hash_map_chaining.cpp

@@ -50,8 +50,8 @@ class HashMapChaining {
                 return pair->val;
             }
         }
-        // 若未找到 key 则返回 nullptr
-        return nullptr;
+        // 若未找到 key 则返回空字符串
+        return "";
     }
 
     /* 添加操作 */

codes/csharp/chapter_hashing/hash_map_chaining.cs

@@ -37,7 +37,7 @@ class HashMapChaining {
     }
 
     /* 查询操作 */
-    public string get(int key) {
+    public string? get(int key) {
         int index = hashFunc(key);
         // 遍历桶，若找到 key 则返回对应 val
         foreach (Pair pair in buckets[index]) {

codes/csharp/chapter_hashing/hash_map_open_addressing.cs

@@ -59,7 +59,7 @@ class HashMapOpenAddressing {
     }
 
     /* 查询操作 */
-    public string get(int key) {
+    public string? get(int key) {
         // 搜索 key 对应的桶索引
         int index = findBucket(key);
         // 若找到键值对，则返回对应 val

codes/python/chapter_hashing/hash_map_chaining.py

@@ -29,7 +29,7 @@ class HashMapChaining:
         """负载因子"""
         return self.size / self.capacity
 
-    def get(self, key: int) -> str:
+    def get(self, key: int) -> str | None:
         """查询操作"""
         index = self.hash_func(key)
         bucket = self.buckets[index]