Update hash_map docs

docs/chapter_hashing/hash_map.md

@@ -512,21 +512,20 @@ $$
     ```typescript title="array_hash_map.ts"
     /* 键值对 Number -> String */
     class Entry {
-
-        public key: number;
-        public val: string;
-
+    public key: number;
+    public val: string;
+    
         constructor(key: number, val: string) {
             this.key = key;
             this.val = val;
         }
     }
-
+    
     /* 基于数组简易实现的哈希表 */
     class ArrayHashMap {
-
-        private readonly bucket: Entry | null[];
-
+    
+        private readonly bucket: (Entry | null)[];
+    
         constructor() {
             // 初始化一个长度为 100 的桶（数组）
             this.bucket = (new Array(100)).fill(null);
@@ -559,8 +558,8 @@ $$
         }
     
         /* 获取所有键值对 */
-        public entries(): Entry[] {
-            let arr = [];
+        public entries(): (Entry | null)[] {
+            let arr: (Entry | null)[] = [];
             for (let i = 0; i < this.bucket.length; i++) {
                 if (this.bucket[i]) {
                     arr.push(this.bucket[i]);
@@ -570,22 +569,22 @@ $$
         }
     
         /* 获取所有键 */
-        public keys(): number[] {
-            let arr = [];
+        public keys(): (number | undefined)[] {
+            let arr: (number | undefined)[] = [];
             for (let i = 0; i < this.bucket.length; i++) {
                 if (this.bucket[i]) {
-                    arr.push(this.bucket[i].key);
+                    arr.push(this.bucket[i]?.key);
                 }
             }
             return arr;
         }
     
         /* 获取所有值 */
-        public values(): string[] {
-            let arr = [];
+        public values(): (string | undefined)[] {
+            let arr: (string | undefined)[] = [];
             for (let i = 0; i < this.bucket.length; i++) {
                 if (this.bucket[i]) {
-                    arr.push(this.bucket[i].val);
+                    arr.push(this.bucket[i]?.val);
                 }
             }
             return arr;