Swap Nodes in Pairs

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.

Naive Way:如果要交换两个指针，必须要知道他们的父指针而非本身。可以使用两个指针，一个每回跳两次，一个每回跳一次，交换他们的子节点。

使用constant space。并且使用了一个额外的头结点方便记录返回值。

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode swapPairs(ListNode head) {
        ListNode newHead = new ListNode(0);
        newHead.next = head;
        ListNode p1 = newHead, p2 = p1;
        while(p2.next!=null){
            p2 = p2.next;
            if(p2.next==null) break;
            p2 = p2.next;
            // switch
            ListNode temp = p1.next;
            p1.next = p2;
            temp.next = p2.next;
            p2.next = temp;
            
            p1 = temp;
            p2 = temp;
        }
        return newHead.next;
    }
}