24. Swap Nodes in Pairs.py

# Definition for singly-linked list.
class ListNode(object):
    def __init__(self, x):
        self.val = x
        self.next = None

class Solution(object):
    def swapPairs_loop(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        if not head or not head.next: return head
        dummy = ListNode(0)
        dummy.next = head
        cur = dummy

        while cur.next and cur.next.next:
            first = cur.next
            second = cur.next.next
            cur.next = second
            first.next = second.next
            second.next = first
            cur = cur.next.next
        return dummy.next

    def swapPairs(self, head):
        #recurse
        if not head or not head.next: return head
        new_start = head.next.next
        head, head.next = head.next, head
        head.next.next = self.swapPairs(new_start)
        return head


node1 = ListNode(1)
node2 = ListNode(2)
node3 = ListNode(3)
node4 = ListNode(4)
node5 = ListNode(5)
node1.next = node2
node2.next = node3
node3.next = node4
node4.next = node5
s = Solution()
s.swapPairs(node1)