LeetCode: Swap Nodes in Pairs

## https://code.dennyzhang.com/swap-nodes-in-pairs
# 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
        """
        ## Idea: pointer Each time move 2 steps
        ## Complexity: Time O(n), Space O(1)
        ## Assumptions:
        ##     One element, returns itself
        ##     If odd number of elements, don't change for the last element
        ## Data Sample:
        ##      1 -> 2 -> 3 -> 4 -> 5 -> 6
        ##                .    .
        ##           p    q    r
        if (head is None) or (head.next is None):
            return head

        # swap the first 2 nodes
        p = head
        q = head.next
        r = q.next

        # change head
        head = q
        q.next = p
        p.next = r

        q = p.next
        while(q and (q.next is not None)):
            r = q.next
            # swap q and r
            tmp = r.next
            p.next = r
            r.next = q
            q.next = tmp
            # move for 2 steps
            p = p.next.next
            q = p.next
        return head