swift 单向链表

节点

class ListNode<T> {
    var value: T
    var next: ListNode?
    
    public init(value: T, next: ListNode? = nil) {
        self.value = value
        self.next = next
    }
}

扩展描述

extension ListNode: CustomStringConvertible {
    var description: String {
        guard let next = next else {
            return "\(value)"
        }
        return "\(value)" + "\(String(describing: next))"
    }
}

链表实现

class LinkedSingleList<T> {
    var head: ListNode<T>?
    var tail: ListNode<T>?
    
    var isEmpty: Bool {
        return head == nil
    }
    
    // 相当于压栈， 后面的元素放到最上(前)面
    func push(_ value: T) {
        head = ListNode(value: value, next: head)
        if tail == nil {
            tail = head
        }
    }
    
    // 出栈
    @discardableResult
    func pop() -> T? {
        trackTailNode()
        defer {
            head = head?.next
            if isEmpty {
                tail = nil
            }
        }
        return head?.value
    }
    
    // 尾部追加
    func append(_ value: T) {
        trackTailNode()
        guard !isEmpty else {
            push(value)
            return
        }
        
        tail?.next = ListNode(value: value)
        // 别忘记尾部指针后移
        tail = tail?.next
    }
    
    // 根据索引找到相关节点
    @discardableResult
    func node(at index: Int) -> ListNode<T>? {
        var currentIndex = 0
        var currentNode: ListNode<T>?
        while currentNode != nil && currentIndex < index {
            currentIndex += 1;
            currentNode = currentNode?.next
        }
        return currentNode
    }
    
    // 删除最后一个节点
    @discardableResult
    func removeLast() -> T? {
        guard let head = head, head.next != nil else {
            return pop()
        }
        var prev = head
        var current = head
        while let next = current.next {
            prev = current
            current = next
        }
        prev.next = nil
        tail = prev
        return current.value
    }
    
    // 删除指定索引的节点
    @discardableResult
    func remove(at index: Int) -> T? {
        guard let head = head, head.next != nil else {
            return pop()
        }
        var prev = head
        var current = head
        var currentIndex = 0
        while let next = current.next, currentIndex <= index {
            prev = current
            current = next
            currentIndex += 1
        }
        prev.next = current.next
        return current.value
    }
    
    // 删除指定节点后的节点
    @discardableResult
    func remove(after node: ListNode<T>) -> T? {
        trackTailNode()
        defer {
            if tail === node.next {
                tail = node
            }
            node.next = node.next?.next
        }
        return node.next?.value
    }
    
    // 反转
    func reverse() {
        var prev = head
        var current = head?.next
        // 头部变尾部
        tail = head
        // 最后一个节点没有next
        tail?.next = nil
        while current != nil {          // 直到 current 为 nil
            let next = current?.next    // 获取下一个节点
            current?.next = prev        // 重新设置下一个节点
            prev = current              // 向前移动
            current = next              // 向后移动
        }
        head = prev
    }
    
    // 找到尾部
    func trackTailNode() {
        guard !isKnownUniquelyReferenced(&head) else { return }
        guard var oldNode = head else { return }
        // 重置 head
        head = ListNode(value: oldNode.value)
        // 初始化变量
        var newNode = head
        // 遍历直到找到尾部节点
        while let nextOldNode = oldNode.next {
            newNode?.next = ListNode(value: nextOldNode.value)
            newNode = newNode?.next
            oldNode = nextOldNode
        }
        tail = newNode
    }
    
    // 打印
    func printInReverse<T>(_ node: ListNode<T>?) {
        guard let node = node else { return }
        printInReverse(node.next)
        print(node.value)
    }
}