[leetcode]24. Swap Nodes in Pairs交换节点对

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

You may not modify the values in the list's nodes, only nodes itself may be changed.

Example:

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

题意：

给定一个链表，每两个节点做一下交换。

Solution1: Two Pointers(fast & slow) 

(1) use Two Pointers(fast & slow)  to find a pair

(2)swap

(3) move forward to find a new pair

code:

 /*
 Time: O(n)
 Space: O(1)
 */
 class Solution {
        public ListNode swapPairs(ListNode head) {
         ListNode dummy = new ListNode(-1);
         dummy.next = head;
         head = dummy;
         while (head.next != null && head.next.next != null) {
             // narrow to find a pair
             ListNode slow = head.next;
             ListNode fast = head.next.next;

             // swap
             head.next = fast;
             slow.next = fast.next;
             fast.next = slow;

             // move to next pair
             head = slow;
         }
         return dummy.next;
     }
 }