1class Node {
2  constructor(val) {
3    this.data = val;
4    this.next = null;
5    this.prev = null;
6  }
7}
8
9class DoublyLinkedList {
10  constructor(val) {
11    this.head = null;
12  }
13  addLast(val) {
14    const newNode = new Node(val);
15    if (!this.head) {
16      this.head = newNode;
17      return;
18    }
19    let current = this.head;
20    let previous = null;
21    while (current.next) {
22      current = current.next;
23    }
24    current.next = newNode;
25    newNode.prev = current;
26    return;
27  }
28  printForwards() {
29    let current = this.head;
30
31    while (current) {
32      console.log(current.data);
33      current = current.next;
34    }
35  }
36  printBackwards() {
37    let current = this.head;
38
39    while (current.next) {
40      current = current.next;
41    }
42
43    while (current) {
44      console.log(current.data);
45      current = current.prev;
46    }
47  }
48  findPair(targetSum) {
49    let currentTail = this.head;
50    let currentHead = this.head;
51    let element = [];
52    while (currentTail.next) {
53      currentTail = currentTail.next;
54    }
55    while (currentHead !== currentTail && currentHead.prev !== currentTail) {
56      let sum = currentHead.data + currentTail.data;
57      if (sum < targetSum) {
58        currentHead = currentHead.next;
59      } else if (sum > targetSum) {
60        currentTail = currentTail.prev;
61      } else {
62        element.push([currentHead.data, currentTail.data]);
63        currentHead = currentHead.next;
64        currentTail = currentTail.prev;
65      }
66    }
67    return element;
68  }
69}
70
71const linkedList = new DoublyLinkedList();
72linkedList.addLast(1);
73linkedList.addLast(2);
74linkedList.addLast(4);
75linkedList.addLast(5);
76linkedList.addLast(6);
77linkedList.addLast(7);
78linkedList.addLast(8);
79linkedList.addLast(9);
80// linkedList.printForwards();
81// linkedList.printBackwards();
82console.log(linkedList.findPair(7));
83