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.
public ListNode swapPairs(ListNode head) {
ListNode Head = new ListNode(0);
Head.next = head;
ListNode pre = Head;
ListNode cur = pre.next;
while (cur!=null&&cur.next!=null){
ListNode next = cur.next.next;
ListNode tail = cur.next;
tail.next = cur;
cur.next = next;
pre.next = tail;
pre = cur;
cur = next;
}
return Head.next;
}