https://leetcode.com/problems/swap-nodes-in-pairs/

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/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} head
 * @return {ListNode}
 */
const swapPairs = head => {
    let dummy = new ListNode()
    dummy.next = head
    let p = dummy
    while(p.next && p.next.next) {
        let x = p.next
        let y = p.next.next
        x.next = y.next
        y.next = x
        p.next = y
        p = p.next.next
    }
    return dummy.next
};