Swap Nodes in Pairs

题目：

Given a linked list, swap every two adjacent nodes and return its head.

Given 1->2->3->4, you should return the list as 2->1->4->3.

分析：

无

解法：

public class Solution {
    /**
     * @param head a ListNode
     * @return a ListNode
     */
    public ListNode swapPairs(ListNode head) {
        // Write your code here
        if (head == null || head.next == null) {
            return head;
        }
        ListNode dummy = new ListNode(-1);
        dummy.next = head;
        ListNode prev = dummy;
        ListNode curr = head;
        while (curr != null && curr.next != null) {
            ListNode next = curr.next;
            curr.next = next.next;
            next.next = prev.next;
            prev.next = next;

            prev = curr;
            curr = curr.next;
        }
        return dummy.next;
    }
}