[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;
    }
}