LeetCode[24]Swap Nodes in Pairs

Description

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

Example:

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

Note:

• Your algorithm should use only constant extra space.
• You may not modify the values in the list’s nodes, only nodes itself may be changed.

Example

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode swapPairs(ListNode head) {
       if(head == null || head.next == null) return head;
        ListNode p = head; //当前节点k
        ListNode q = head.next; //下一节点k+1
        ListNode r = null; //记录上一节点k-1
        head = q;
        while(p != null && q != null) {
        	p.next = q.next; //交换next
        	q.next = p;
        	if(r != null) r.next = q; //更新上一节点的next
        	r = p; //更新节点
        	p = p.next;
        	if(p != null) q = p.next;
        		
        }
        return head;
    }
}