24. Swap Nodes in Pairs

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.

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None
"""
解法：
1.长度小于2直接返回
2.设置一个前置节点和虚拟节点，前置节点负责连接交换过后的节点
3.遍历，相邻的2个节点交换，若只剩下1个结点，不做处理
"""

class Solution(object):
    def swapPairs(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        
        if not head or not head.next:
            return head
        
        cur = head
        dummy= pre = ListNode(None)
        while cur and cur.next:
            tmp = cur.next
            cur.next = cur.next.next
            tmp.next = cur
            pre.next = tmp
            pre = pre.next.next
            cur = cur.next
        return dummy.next