Swap_Nodes_in_Pairs.md

024. Swap Nodes in Pairs

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.

Method:

# 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
        """
        dummy=ListNode(0)
        dummy.next=head
        h=dummy
        if not dummy.next:
            return h.next
        while dummy.next.next:
            n=dummy.next.next
            dummy.next.next=dummy.next.next.next
            n.next=dummy.next
            dummy.next=n
            dummy=dummy.next.next
            if not dummy.next:
                return h.next
        return h.next

Solution:

shorter:

# 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
        """
        dummy=ListNode(0)
        dummy.next=head
        h=dummy
        while dummy.next and dummy.next.next:
            n=dummy.next.next
            dummy.next.next=dummy.next.next.next
            n.next=dummy.next
            dummy.next=n
            dummy=dummy.next.next
        return h.next