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Swap list nodes in pairs challenge

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Problem statement
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.

Full code

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

    def __str__(self):
        head = self
        ans = ''
        while head != None:
            ans = ans + str(head.val)
            head = head.next
            if head != None:
                ans = ans + '->'

        return ans

class Solution(object):
    def swapPairs(self, head):
        :type head: ListNode
        :rtype: ListNode
        start = head

        if head != None and head.next != None:
            start = head.next
        while head != None and head.next != None:
            tmp = head.next
            head.next = tmp.next
            if tmp.next != None and tmp.next.next != None:
                head.next = tmp.next.next
            new_head = tmp.next
            tmp.next = head
            head = new_head
        return start

def main():
    start = ListNode(1)
    tmp = start
    for x in range(2, 9):
        tmp.next = ListNode(x)
        tmp = tmp.next
    solution = Solution()

if __name__ == "__main__": main()