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} */ let 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 };

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