设计链表

Posted on Apr 02, 2025 By ᵇᵒ

Design Linked List

LeetCode 707

Design your implementation of the linked list. You can choose to use a singly or 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 the MyLinkedList class:

  • MyLinkedList() Initializes the MyLinkedList object.
  • int get(int index) Get the value of the \(index^{th}\) node in the linked list. If the index is invalid, return -1.
  • void addAtHead(int 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.
  • void addAtTail(int val) Append a node of value val as the last element of the linked list.
  • void addAtIndex(int index, int val) Add a node of value val before the \(index^{th}\) node in the linked list. If index equals the length of the linked list, the node will be appended to the end of the linked list. If index is greater than the length, the node will not be inserted.
  • void deleteAtIndex(int index) Delete the \(index^{th}\) node in the linked list, if the index is valid.

Example 1:

Input: 
["MyLinkedList", "addAtHead", "addAtTail", "addAtIndex", "get", "deleteAtIndex", "get"]
[[], [1], [3], [1, 2], [1], [1], [1]]

Output: 
[null, null, null, null, 2, null, 3]

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

Constraints:

  • 0 <= index, val <= 1000
  • Please do not use the built-in LinkedList library.
  • At most 2000 calls will be made to get, addAtHead, addAtTail, addAtIndex and deleteAtIndex.

Singly linked list：

class MyLinkedList() {
    private var head: Node? = null
    private var length: Int = 0

    fun get(index: Int): Int {
        return getNodeAt(index)?.value ?: -1
    }

    fun addAtHead(value: Int) {
        addAtIndex(0, value)
    }

    fun addAtTail(value: Int) {
        addAtIndex(length, value)
    }

    fun addAtIndex(index: Int, value: Int) {
        if (index !in 0..length) return
        val node = getNodeAt(index - 1)
        if (node == null) {
            head = Node(value).apply { next = head }
        } else {
            node.next = Node(value).apply { next = node.next }
        }
        length++
    }

    fun deleteHead() {
        deleteAtIndex(0)
    }

    fun deleteTail() {
        deleteAtIndex(length - 1)
    }

    fun deleteAtIndex(index: Int) {
        if (index !in 0..<length) return
        val node = getNodeAt(index - 1)
        if (node == null) {
            head = head?.next
        } else {
            node.next = node.next?.next
        }
        length--
    }

    private fun getNodeAt(index: Int): Node? {
        if (index !in 0..<length) return null
        var node = head
        repeat(index) { node = node?.next }
        return node
    }

    private class Node(val value: Int) {
        var next: Node? = null
    }
}

Doubly linked list:

class MyLinkedList() {
    private var head: Node? = null
    private var tail: Node? = null
    private var length: Int = 0

    fun get(index: Int): Int {
        return getNodeAt(index)?.value ?: -1
    }

    fun addAtHead(value: Int) {
        val node = Node(value)
        node.next = head
        head?.prev = node
        head = node
        if (length == 0) tail = head
        length++
    }

    fun addAtTail(value: Int) {
        val node = Node(value)
        node.prev = tail
        tail?.next = node
        tail = node
        if (length == 0) head = tail
        length++
    }

    fun addAtIndex(index: Int, value: Int) {
        if (index !in 0..length) return
        if (index == 0) {
            addAtHead(value)
        } else if (index == length) {
            addAtTail(value)
        } else {
            val node = Node(value)
            val nodeBeforeIndex = getNodeAt(index - 1)
            node.next = nodeBeforeIndex?.next
            node.prev = nodeBeforeIndex
            nodeBeforeIndex?.next?.prev = node
            nodeBeforeIndex?.next = node
            length++
        }
    }

    fun deleteHead() {
        deleteAtIndex(0)
    }

    fun deleteTail() {
        deleteAtIndex(length - 1)
    }

    fun deleteAtIndex(index: Int) {
        if (index !in 0..<length) return
        val node = getNodeAt(index)
        node?.prev?.next = node?.next
        node?.next?.prev = node?.prev
        if (head == node) head = node?.next
        if (tail == node) tail = node?.prev
        length--
    }

    private fun getNodeAt(index: Int): Node? {
        if (index !in 0..<length) return null
        if (index < length.shr(1)) {
            var node = head
            repeat(index) { node = node?.next }
            return node
        } else {
            var node = tail
            repeat(length - index - 1) { node = node?.prev }
            return node
        }
    }

    private class Node(val value: Int) {
        var next: Node? = null
        var prev: Node? = null
    }
}