leetcode: 24. Swap Nodes in Pairs

# 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.

class ListNode():
    def __init__(self, x):
        self.val = x
        self.next = None

class Solution():
    def swapPairs(self, x):
        cur = dummy = ListNode(0)
        tmp, step = None, 1
        while x:
            if step % 2:
                tmp = ListNode(x.val)
            else:
                cur.next, cur.next.next = ListNode(x.val), tmp
                cur, tmp = cur.next.next, None
            x, step = x.next, step + 1
        if tmp:
            cur.next = tmp
        return dummy.next


if __name__ == "__main__":
    x, x.next, x.next.next, x.next.next.next = ListNode(1), ListNode(2), ListNode(3), ListNode(4)
    res = Solution().swapPairs(x)
    assert '{0}->{1}->{2}->{3}'.format(res.val, res.next.val, res.next.next.val, res.next.next.next.val) \
           == '2->1->4->3'