public Integer dequeue() { if (isEmpty()) { returnnull; } Integervalue= values[front]; front = (front + 1) % values.length; // 同样实现循环队列 counter--; return value; }
最后，对于 displayQueue 方法，这个根据个人爱好实现即可，这里依旧提供一种方法：
publicvoiddisplayQueue() { if (isEmpty()) { System.out.println("Empty queue!"); return; }
System.out.print("front-> "); for (inti= front; i <= rear; i++) { if (i != front) System.out.print("\t"); if (i == values.length) { i = 0; } else { System.out.printf("|\t%7.4f |", values[i]); if (i != values.length - 1) System.out.print("\n"); } } System.out.println("<-rear"); }
最后将上面的方法整合到 Queue 类中即可，有了链表的基础，实现起来还是不难的。
本文当然不会只是介绍实现方法，有请assignment登场。
链表、栈、队列的进阶
链表的进阶
定义以下三个方法：
removeNodes
removeDuplicates
reverseList
其中
public int removeNodes(double x)
Removes all the elements from a linked list that have value x. (从链表中删除所有值为 x 的元素)
Returns the number of occurrences of x. (返回 x 出现的次数)
public void removeDuplicates()
Remove duplicates from sorted list (you may assume that the node values in the list are in non-decremental order). (从排序链表中删除重复元素（你可以假设链表中的节点值是非递减顺序的）)
Removes all nodes that have duplicate values, leaving only nodes with distinct values. (删除所有具有重复值的节点，只保留具有不同值的节点)
Your implementation should finish the removing with SINGLE traversal of the whole list, which means you cannot simply invoke the removeNodes method for removing the duplicates. (你的实现应该在对整个列表进行单次遍历的情况下完成删除，这意味着你不能简单地调用 removeNodes 方法来删除重复项)
public void reverseList()
Reverse the linked list WITHOUT using extra space. (反转链表不使用额外空间)
Hint: Reverse the linked list can be performed by modifying the “next” pointer of current node from the original next node to its previous node. (提示：反转链表可以通过将当前节点的“下一个”指针从原始下一个节点修改为其前一个节点来执行)
if (head == null) { return; } // define head and second node is null Nodedummy=newNode(0); dummy.setNext(head); NodepastNode= dummy; NodecurrNode= head; booleanhasDuplicate=false;
while (currNode != null) {// condition: when head is not null return null NodenextNode= currNode.getNext(); if (nextNode != null && currNode.getData() == nextNode.getData()) { hasDuplicate = true; currNode = nextNode; } else { if (hasDuplicate) { pastNode.setNext(nextNode); hasDuplicate = false; } else { pastNode = currNode; } currNode = nextNode; } } head = dummy.getNext(); }
对于 reverseList 方法，我们可以使用三个指针来反转链表：pastNode 指向前一个节点，currNode 指向当前节点，nextNode 指向下一个节点。在遍历链表时，我们将当前节点的 next 指针指向前一个节点，然后更新指针以继续遍历。最后，将头节点更新为 prevNode，即新的头节点。
postfix is a string representing a valid postfix expression which contains only digits and ‘+’, ‘-’, ‘*’, ‘/’ operators, separated by ‘,’. (后缀表达式是一个字符串，表示一个有效的后缀表达式，只包含数字和‘+’，‘-’，‘*’，‘/’运算符，以‘，’分隔)
You can assume the length of postfix will not exceed 50. (你可以假设后缀表达式的长度不会超过50)
Returns the evaluation result. (返回计算结果)
Consider the postfix expression evaluation algorithm introduced in Lec3:
If the element is an operand, push it to stack. (如果元素是操作数，则将其压入栈中)
If the element is an operator O, pop twice and get A and B respectively. Calculate BOA and push it back to stack. (如果元素是运算符 O，则弹出两次，分别获取 A 和 B。计算 BOA 并将其压回栈中)
When the expression is ended, the value in the stack is the final answer. (当表达式结束时，栈中的值即为最终答案)
CircularDeque(int size) Initializes the deque with a maximum size of size. (使用最大大小为 size 初始化双端队列)
Integer insertFront() Adds an item at the front of Deque. Returns this item if the operation is successful, or Null otherwise. (在队列前端插入一个元素。如果操作成功，返回该元素，否则返回 Null。)
Integer insertLast() Adds an item at the rear of Deque. Returns this item if the operation is successful, or Null otherwise. (在队列后端插入一个元素。如果操作成功，返回该元素，否则返回 Null。)
Integer deleteFront() Deletes an item from the front of Deque. Returns this item if the operation is successful, or Null otherwise. (从队列前端删除一个元素。如果操作成功，返回该元素，否则返回 Null。)
Integer deleteLast() Deletes an item from the rear of Deque. Returns this item if the operation is successful, or Null otherwise. (从队列后端删除一个元素。如果操作成功，返回该元素，否则返回 Null。)
boolean isEmpty() Returns true if the deque is empty, or false otherwise. (如果双端队列为空，返回 true，否则返回 false。)
boolean isFull() Returns true if the deque is full, or false otherwise. (如果双端队列已满，返回 true，否则返回 false。)
