fe-program.js

class Node {
  constructor(value) {
    this.value = value;
    this.next = null;
  }
}

class LinkedList {
  constructor(value) {
    const newNode = new Node(value);
    this.head = newNode;
    this.tail = this.head;
    this.length = 1;
  }
  push(value) {
    const newNode = new Node(value);
    if (!this.head) {
      this.head = newNode;
      this.tail = newNode;
    } else {
      this.tail.next = newNode;
      this.tail = newNode;
    }
    this.length++;
    return this;
  }
  pop() {
    if (!this.head) {
      return undefined;
    }
    let temp = this.head;
    let pre = this.head;
    while (temp.next) {
      pre = temp;
      temp = temp.next;
    }
    this.tail = pre;
    this.tail.next = null;
    this.length--;
    if (this.length === 0) {
      this.head = null;
      this.tail = null;
    }
    return temp;
  }

  unShift(value) {
    const newNode = new Node(value);
    if (!this.head) {
      this.head = newNode;
      this.tail = newNode;
    } else {
      newNode.next = this.head;
      this.head = newNode;
    }
    this.length++;
    return this;
  }
  shift() {
    if (!this.head) return undefined;
    let temp = this.head;
    this.head = this.head.next;
    temp.next = null;
    this.length--;
    if (this.length === 0) {
      this.tail = null;
    }
    return temp;
  }
}

let myLinkedList = new LinkedList(4);
myLinkedList.unShift(5);
myLinkedList.push(9);
myLinkedList.pop();
myLinkedList.shift();
console.log(myLinkedList);

// get center value
class ListNode {
  constructor(value = 0, next = null) {
    this.value = value;
    this.next = next;
  }
}

function findCenterElement(head) {
  if (head === null) {
    return null;
  }

  let slow = head;
  let fast = head;

  while (fast !== null && fast.next !== null) {
    slow = slow.next;
    fast = fast.next.next;
  }
  return slow.value;
}

let node1 = new ListNode(1);
let node2 = new ListNode(2);
let node3 = new ListNode(3);
let node4 = new ListNode(4);
let node5 = new ListNode(5);

node1.next = node2;
node2.next = node3;
node3.next = node4;
node4.next = node5;

console.log(findCenterElement(node1));