Reverse pairs of elements in a singly-linked list.
Examples
L = null, after reverse is null
L = 1 -> null, after reverse is 1 -> null
L = 1 -> 2 -> null, after reverse is 2 -> 1 -> null
L = 1 -> 2 -> 3 -> null, after reverse is 2 -> 1 -> 3 -> null
class Solution(object):
def reverseInPairs(self, head):
if not head or not head.next:
return head
next_node = head.next
head.next = self.reverseInPairs(head.next.next)
next_node.next = head
return next_node
