Given a linked list, swap every two adjacent nodes and return its head.
Example:
Given 1->2->3->4, you should return the list as 2->1->4->3.
Note:
Your algorithm should use only constant extra space.
You may not modify the values in the list’s nodes, only nodes itself may be changed.
递归和迭代:
public ListNode swapPairs(ListNode head) {
if(head == null || head.next == null) return head;
ListNode node = head.next;
head.next = swapPairs(head.next.next);
node.next = head;
return node;
}
public ListNode swapPairs(ListNode head) {
ListNode dummy = new ListNode(0);
dummy.next = head;
ListNode cur = dummy;
while(cur.next != null && cur.next.next != null){
ListNode first = cur.next;
ListNode second = cur.next.next;
first.next = second.next;
second.next = first;
cur.next = second;
cur = cur.next.next;
}
return dummy.next;
}