24. Swap Nodes in Pairs ***

题目要求：

Given a linked list, swap every two adjacent nodes and return its head.

Examples:

Given 1->2->3->4, you should return the list as 2->1->4->3.

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Your algorithm should use only constant space. You may not modify the values in the list, only nodes itself can be changed.

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解题思路：

• 画图解题

# Time:  O(n)
# Space: O(1)
class ListNode:
    def __init__(self, x):
        self.val = x
        self.next = None

    def __repr__(self):
        if self:
            return "{} -> {}".format(self.val, self.next)
class Solution:
    # @param a ListNode
    # @return a ListNode
    def swapPairs(self, head):
        dummy = ListNode(0)
        dummy.next = head
        current = dummy

        while current.next and current.next.next:
            next_one, next_two, next_three = current.next, current.next.next, current.next.next.next
            current.next = next_two
            next_two.next = next_one
            next_one.next = next_three
            current = next_one
        return dummy.next