24. Swap Nodes in Pairs

/**
 * @param {ListNode} head
 * @return {ListNode}
 */

const { ListNode, arrToLL, LLToArr } = require('./utils.js')
var swapPairs = function (head) {
	if (!head || !head.next) return head
	let node1 = head,
		node2 = head.next,
		newHead = node2,
		prev,
		rest

	while (node2) {
		rest = node2.next
		node2.next = node1
		node1.next = rest

		if (prev) prev.next = node2
		prev = node1
		node1 = rest
		if (!node1) return newHead
		node2 = node1.next
	}
	return newHead
}

//Test cases
console.log(LLToArr(swapPairs(arrToLL([1, 2, 3, 4, 5]))))
console.log(LLToArr(swapPairs(arrToLL([1, 2, 3, 4]))))
console.log(LLToArr(swapPairs(arrToLL([1, 2, 3]))))
console.log(LLToArr(swapPairs(arrToLL([1, 2]))))
console.log(LLToArr(swapPairs(arrToLL([1]))))
console.log(LLToArr(swapPairs(arrToLL([]))))