24. Swap Nodes in Pairs.js

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var swapPairs = function(head) {
    if(head === null || head.next === null) return head;
    let ans = new ListNode(0);
    ans.next = head;
    let curr = ans;
    while(curr.next !== null && curr.next.next !== null){
        let t1 = curr.next;
        let t2 = curr.next.next;
        curr.next = t2;
        t1.next = t2.next;
        t2.next = t1;
        curr = curr.next.next;
    }
    return ans.next;
};