LeetCode-24 - Swap Nodes in Pair

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.

Solution. 递归

应该很容易想到用递归来分解计算步骤：

a. 先交换最前面两个元素
b. 再算后面的List
c. 将a/b两步结果接起来

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

class Solution(object):
    def swapPairs(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        if not head or not head.next:
            return head
        _head = head.next
        head.next = self.swapPairs(_head.next)
        _head.next = head
        return _head