LeetCode 24 Swap Nodes in Pairs

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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.

Python Code

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# Definition for singly-linked list.
# 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 head == None or head.next == None:
            return head
        ret = head.next
        i = head
        j = head.next
        pre = None
        while j != None:
            i.next = j.next
            j.next = i
            if pre != None:
                pre.next = j
            if i.next != None:
                pre = i
                i = i.next
                j = i.next
            else:
                break
        return ret