题意：给定一个链表，要求两两交换节点，返回头节点。

24. Swap Nodes in Pairs (opens new window)

比较简单的链表操作，两两交换即可。

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode swapPairs(ListNode head) {
        // 在head前添加一个节点，方便后续的操作
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode prev = dummy;
        ListNode left = head;
        ListNode right;
        // a -> b -> c -> d
        // prev left right
        // a -> c -> b -> d
        while (left != null && left.next != null) {
            right = left.next;
            prev.next = right;
            left.next = right.next;
            right.next = left;
            prev = left;
            left = left.next;
        }
        return dummy.next;
    }
}