Swap Nodes in Pairs

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

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

Your algorithm should use only constant space. You may?not?modify the values in the list, only nodes itself can be changed.

?

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode swapPairs(ListNode head) {
    	if (head==null || head.next==null) {
    		return head;
    	}
    	ListNode first = null;
    	ListNode second = head;
    	ListNode third = head.next;
    	while (third != null) {
    		ListNode h = swap(second, third);
    		if (first == null) {
    			head = h;
    		} else {
    			first.next = h;
    		}
    		ListNode tmp = second;
    		second = third;
    		third = tmp;
    		if (third.next == null) {
    			return head;
    		} else {
    			first = third;
    			second = second.next.next;
    			third = third.next.next;
    		}
    	}
    	return head;
    }

	private ListNode swap(ListNode second, ListNode third) {
		second.next = third.next;
		third.next = second;
		return third;
	}
}

?

Swap Nodes in Pairs

原文：http://hcx2013.iteye.com/blog/2216804