链表的操作

class List {
    constructor(headValue) {
        this.head = headValue ? this.createNode(headValue) : null
        this.length = headValue ? 1 : 0
    }

    createNode(value) {
        return {
            value,
            next: null
        }
    }

    // 末尾加
    add(value) {
        let current = this.head
        while(current.next) {
            current = current.next
        }
        current.next = this.createNode(value)
        this.length = this.length + 1
    }

    // 是否存在某个节点
    exist(value) {
        let current = this.head
        let index = 1
        while(current) {
            if (current.value === value) return index
            index ++
            current = current.next
        }

        return false
    }

    // 查找某位的值
    findByIndex(index) {
        if (index === 1) return this.head
        if (index > this.length) return null

        let current = this.head
        while (index > 1) {
            current = current.next
            index--
        }

        return current
    }

    // 删 根据index
    delByIndex(index) {
        if (this.head === null || index < 1 || index > this.length) return null
        
        this.length = this.length - 1
        if (index === 1) {
            // 删除第一位
            this.head = this.head.next
            return true
        }
        const pre = this.findByIndex(index - 1)
        pre.next = pre.next.next
        return true
    }

    // del 根据值
    delByValue(value) {
        const index = this.exist(value)
        if (index === false) return index

        return this.delByIndex(index)
    }
    // 插入
    insert(index, value) {
        
        if (index === 0) {
            const newNode = this.createNode(value)
            newNode.next = this.head
            this.head = newNode
            this.length = this.length + 1
            return true
        }

        if (index > this.length) return false

        const now = this.findByIndex(index)
        const node = this.createNode(value)
        node.next = now.next
        now.next = node
        this.length++
        return

    }

    // 反转， 三个指针（next res current）
    reverseList() {
        if (this.head === null || this.head.next === null) return this.head

        let res = null
        let next;
        let current = this.head

        while (current) {
            next = current.next
            current.next = res
            res = current
            current = next
        }

        return res
    }

    // 是否存在环
    isCircle() {
        if (this.length < 2) return false
        
        let fast = this.head
        let slow = this.head

        while(fast && fast.next) {
            fast = fast.next.next 
            slow = slow.next

            if (fast === slow) return true
        }

        return false
    }
    
    back(index) {
        if (index === this.length) return this.head
        if (index < 1 || index > this.length) return null

        let fast = this.head
        let slow = this.head

        while(index--) {
            fast = fast.next
        }

        while(fast) {
            fast = fast.next
            slow = slow.next
        }

        return slow


    }
}