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24 Swap Nodes in Pairs Leetcode Problem Javascript Solution

Swap Nodes in Pairs
Medium
Given a linked list, swap every two adjacent nodes and return its head.
You may not modify the values in the list's nodes, only nodes itself may be changed.

Example:
Given 1->2->3->4, you should return the list as 2->1->4->3.
class ListNode {
  constructor(val, next=null) {
    this.val = val;
    this.next = next;
  }
}

var swapPairs = function(head) {

  const prehead = new ListNode(null, head);

  for (let p = prehead; p.next !== null && p.next.next !== null;) {
    const p1 = p.next;
    const p2 = p1.next; // 0 1 2 3 4
    p1.next = p2.next; // 0 1 3 4
    p2.next = p1; // 2 1 3 4
    p.next = p2; // 0 2 1 3 4
    p = p1;
  }

  return prehead.next
};

console.log(swapPairs(new ListNode(1, new ListNode(2, new ListNode(3, new ListNode(4))))));

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