LeetCode刷题笔录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.

这题想法和Merge Two Sorted Linked List类似，也是建一个fakeHead，然后一个一个把原来链表里的node插到新链表里。

比如这个1->2，p1=1。那么dummyP.next=2, nextHead=p1.next.next. dummyP.next.next = 1.最后p1.next=null。一定要记住最后一步，要不然新链表就是2121212121....没完没了了

public class Solution {
    public ListNode swapPairs(ListNode head) {
        ListNode fakeHead = new ListNode(0);
        ListNode dummyP = fakeHead;
        ListNode p1 = head;
        ListNode nextHead;
        while(p1 != null){
            if(p1.next == null){
                dummyP.next = p1;
                break;
            }
            nextHead = p1.next.next;
            dummyP.next = p1.next;
            p1.next.next = p1;
            //must set p1.next = null to avoid self loops
            p1.next = null;
            p1 = nextHead;
            dummyP = dummyP.next.next;
        }
        return fakeHead.next;
    }
}