LeetCode

题目：

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.

思路：

保持三个指针，往前推进，不断变换

package list;

public class SwapNodesInPairs {

    public ListNode swapPairs(ListNode head) {
        ListNode x = new ListNode(0);
        x.next = head;
        ListNode y = head;
        ListNode z = null;

        ListNode first = x;
        
        while (y != null && y.next != null) {
            z = y.next;
            ListNode tmp = z.next;
            x.next = z;
            z.next = y;
            y.next = tmp;
            
            x = y;
            y = tmp;
        }
        
        return first.next; 
    }
    
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        ListNode a1 = new ListNode(1);
        ListNode a2 = new ListNode(2);
        ListNode a3 = new ListNode(3);
        ListNode a4 = new ListNode(4);
        ListNode a5 = new ListNode(5);
        a1.next = a2;
        a2.next = a3;
        a3.next = a4;
        a4.next = a5;
        a5.next = null;
        
        SwapNodesInPairs s = new SwapNodesInPairs();        
        ListNode x = s.swapPairs(a1);
        while (x != null) {
            System.out.println(x.val);
            x = x.next;
        }
    }

}