LeetCode 24. Swap Nodes in Pairs Java写法

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.
• 
  

/**Java主要注意边界问题就好了

/**
 * 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 p1=head;
        ListNode p2=head.next;
        while(p2!=null){
            int t=p2.val;
            p2.val=p1.val;
            p1.val=t;
            if(p1.next!=null&&p1.next.next!=null)
            {p1=p1.next.next;
            }else
                break;
            if(p2.next!=null&&p2.next.next!=null)
            {p2=p2.next.next;}
            else
                break;
        }
        return head;
    }
}

递归方法

class Solution {
    public ListNode swapPairs(ListNode head) {
        if(head==null||head.next==null)
            return head;
        ListNode p1=head.next;
        ListNode p2=p1.next;
        p1.next=head;
        head.next=swapPairs(p2);
        return p1;
    }
}