Free memory after removing

a node from a LinkedList or TreeNode.

docs/chapter_stack_and_queue/queue.md

@@ -331,7 +331,10 @@ comments: true
         int poll() {
             int num = peek();
             // 删除头结点
+            ListNode *tmp = front;
             front = front->next;
+            // 释放内存
+            delete tmp; 
             queSize--;
             return num;
         }

docs/chapter_stack_and_queue/stack.md

@@ -311,7 +311,10 @@ comments: true
         /* 出栈 */
         int pop() {
             int num = top();
+            ListNode *tmp = stackTop;
             stackTop = stackTop->next;
+            // 释放内存
+            delete tmp;
             stkSize--;
             return num;
         }