LeetCode | Swap Nodes in Pairs

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.
分析
按部就班，用个哨兵简化代码。

代码

public class SwapNodesInPairs {
public ListNode swapPairs(ListNode head) {
ListNode dummy = new ListNode(0);
dummy.next = head;

ListNode pPre = dummy;
ListNode p = head;
while (p != null && p.next != null) {
// swap
pPre.next = p.next;
p.next = p.next.next;
pPre.next.next = p;

// move
pPre = p;
p = pPre.next;
}

return dummy.next;
}
}