24. Swap Nodes in Pairs

Problem:

Given a linked list, swap every two adjacent nodes and return its head.

Example:

Given 1->2->3->4, you should return the list as 2->1->4->3.

Note:

• Your algorithm should use only constant extra space.
• You may not modify the values in the list’s nodes, only nodes itself may be changed.

Solution:

1. Draw the nodes down on paper to reason about the relationships.
2. Pointing to every active node is an easy way to keep on track.

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var swapPairs = function(head) {
  const prehead = { next: head }

  for (let p = prehead; p.next !== null && p.next.next !== null;) {
    const p1 = p.next
    const p2 = p1.next
    p1.next = p2.next
    p2.next = p1
    p.next = p2
    p = p1
  }

  return prehead.next
};