707. Design Linked List

1. 原始题目

Design your implementation of the linked list. You can choose to use the singly linked list or the doubly linked list. A node in a singly linked list should have two attributes: val and next. val is the value of the current node, and next is a pointer/reference to the next node. If you want to use the doubly linked list, you will need one more attribute prev to indicate the previous node in the linked list. Assume all nodes in the linked list are 0-indexed.

Implement these functions in your linked list class:

• get(index) : Get the value of the index-th node in the linked list. If the index is invalid, return -1.
• addAtHead(val) : Add a node of value val before the first element of the linked list. After the insertion, the new node will be the first node of the linked list.
• addAtTail(val) : Append a node of value val to the last element of the linked list.
• addAtIndex(index, val) : Add a node of value val before the index-th node in the linked list. If index equals to the length of linked list, the node will be appended to the end of linked list. If index is greater than the length, the node will not be inserted.
• deleteAtIndex(index) : Delete the index-th node in the linked list, if the index is valid.

Example:

MyLinkedList linkedList = new MyLinkedList();
linkedList.addAtHead(1);
linkedList.addAtTail(3);
linkedList.addAtIndex(1, 2);  // linked list becomes 1->2->3
linkedList.get(1);            // returns 2
linkedList.deleteAtIndex(1);  // now the linked list is 1->3
linkedList.get(1);            // returns 3

Note:

• All values will be in the range of [1, 1000].
• The number of operations will be in the range of [1, 1000].
• Please do not use the built-in LinkedList library.

2. 题目理解

设计链表的实现。您可以选择使用单链表或双链表。单链表中的节点应该具有两个属性：val 和 next。val 是当前节点的值，next 是指向下一个节点的指针/引用。如果要使用双向链表，则还需要一个属性 prev 以指示链表中的上一个节点。假设链表中的所有节点都是 0-index 的。

在链表类中实现这些功能：

• get(index)：获取链表中第 index 个节点的值。如果索引无效，则返回-1。
• addAtHead(val)：在链表的第一个元素之前添加一个值为 val 的节点。插入后，新节点将成为链表的第一个节点。
• addAtTail(val)：将值为 val 的节点追加到链表的最后一个元素。
• addAtIndex(index,val)：在链表中的第 index 个节点之前添加值为 val  的节点。如果 index 等于链表的长度，则该节点将附加到链表的末尾。如果 index 大于链表长度，则不会插入节点。
• deleteAtIndex(index)：如果索引 index 有效，则删除链表中的第 index 个节点。

示例：

MyLinkedList linkedList = new MyLinkedList();
linkedList.addAtHead(1);
linkedList.addAtTail(3);
linkedList.addAtIndex(1,2);   //链表变为1-> 2-> 3
linkedList.get(1);            //返回2
linkedList.deleteAtIndex(1);  //现在链表是1-> 3
linkedList.get(1);            //返回3

提示：

• 所有值都在 [1, 1000] 之内。
• 操作次数将在  [1, 1000] 之内。
• 请不要使用内置的 LinkedList 库。

注意·的点：空链表，索引范围为0~超出链表长度~我改了两次才通过。

3. 解题

 class ListNode:          # 定义结点类型
      def __init__(self, x):
          self.val = x
          self.next = None

 class MyLinkedList:

     def __init__(self):
         """
         Initialize your data structure here.
         """
         self.head = None     # 初始化一个头结点为空

     def get(self, index: int) -> int:      # 返回第index个结点，若index不合法则返回-1
         """
         Get the value of the index-th node in the linked list. If the index is invalid, return -1.
         """
         if self.head == None:     # 空链表对于所有index都不合法
             return -1
         p = self.head
         for i in range(index):
             if not p.next:
                 return -1
             p = p.next
         return p.val

     def addAtHead(self, val: int) -> None:     # 在头部插入一个结点
         """
         Add a node of value val before the first element of the linked list. After the insertion, the new node will be the first node of the linked list.
         """
         if self.head == None:           # 对空链表直接将该结点赋予头结点即可
             self.head =  ListNode(val)
         else:
             new_node = ListNode(val)
             new_node.next = self.head
             self.head = new_node          # 更新头结点

     def addAtTail(self, val: int) -> None:       # 在尾部插入一个结点
         """
         Append a node of value val to the last element of the linked list.
         """
         if self.head == None:             # 对空链表直接将该结点赋予头结点即可
             self.head = ListNode(val)
         p = self.head
         while(p.next):
             p = p.next
         p.next = ListNode(val)

     def addAtIndex(self, index: int, val: int) -> None:    # 在第index位置插入结点
         """
         Add a node of value val before the index-th node in the linked list. If index equals to the length of linked list, the node will be appended to the end of linked list. If index is greater than the length, the node will not be inserted.
         """
         p = self.head
         new_node = ListNode(val)
         if not self.head:        # 若为空链表，除非index=0，才将其作为头结点，否则一切index都不合法
             if index==0:
                 new_node.next = self.head
                 self.head = new_node
             return None
         for i in range(index-1):
             if not p.next:
                 return None
             p = p.next
         new_node.next = p.next
         p.next = new_node

     def deleteAtIndex(self, index: int) -> None:        # 删除index位置的结点
         """
         Delete the index-th node in the linked list, if the index is valid.
         """
         if not self.head:            # 空链表退出
             return None
         if index == 0:               # 头结点单独考虑，直接将头结点赋予下一个结点即可
             self.head = self.head.next
             return None
         p = self.head
         for i in range(index-1):
             if not p.next:
                 return None
             p = p.next
         if p.next:
             p.next = p.next.next

4. 验证

验证之前可以写一个print函数打印当前链表情况：写到类里面去

    def printlist(self):
        p = self.head
        while(p):
            print(p.val,end=' ')
            p = p.next
        print('\n') 

测试：

linkedList = MyLinkedList()

linkedList.addAtHead(5)
linkedList.printlist()

linkedList.addAtHead(2)
linkedList.printlist()

linkedList.deleteAtIndex(1)
linkedList.printlist()

linkedList.addAtIndex(1,9)
linkedList.printlist()

linkedList.addAtHead(4)
linkedList.printlist()

linkedList.addAtHead(9)
linkedList.printlist()

linkedList.addAtHead(8)
linkedList.printlist()

print(linkedList.get(3) )

linkedList.addAtTail(1)
linkedList.printlist()

linkedList.addAtIndex(3,6)
linkedList.printlist()

linkedList.addAtHead(3)
linkedList.printlist()

5

2 5

2

2 9

4 2 9

9 4 2 9

8 9 4 2 9

2
8 9 4 2 9 1

8 9 4 6 2 9 1

3 8 9 4 6 2 9 1