LeetCode #24 Swap Nodes in Pairs

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Problem

Given a linked list, swap every two adjacent nodes and return its head.

Example

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Given 1->2->3->4, you should return the list as 2->1->4->3.

Note

Can solve this problem by using only constant extra space.

Solution

Method: Iterative

Time Complexity: O(n)

Space Complexity: O(1)

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

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

        dummy = ListNode(None)
        dummy.next = head
        pre = dummy
        left = head
        right = head.next
        
        while left and right:
            left.next = right.next
            pre.next = right
            right.next = left
            pre = pre.next.next
            left = pre.next
            if left:
                right = left.next
            else:
                right = None
        
        return dummy.next

Reference

7-8 lines C++ / Python / Ruby