24. Swap Nodes in Pairs
MediumLeetCodeGiven a linked list, swap every two adjacent nodes and return its head. You must solve the problem without modifying the values in the list's nodes (i.e., only nodes themselves may be changed.)
Example 1
Input: head = [1,2,3,4]
Output: [2,1,4,3]
Example 2
Input: head = []
Output: []
Example 3
Input: head = [1]
Output: [1]
Example 4
Input: head = [1,2,3]
Output: [2,1,3]
Constraints:
- The number of nodes in the list is in the range [0, 100]
- 0 <= Node.val <= 100
How to solve the problem
Code
Python
# Definition for singly-linked list.
# class ListNode:
# def __init__(self, val=0, next=None):
# self.val = val
# self.next = next
class Solution:
def swapPairs(self, head: Optional[ListNode]) -> Optional[ListNode]:
dummyhead = ListNode(0, head)
cur = dummyhead
while(cur.next!= None and (cur.next).next!= None):
# Save node1, node3
node1 = cur.next
node3 = ((cur.next).next).next
# Swapping
cur.next = (cur.next).next
(cur.next).next = node1
node1.next = node3
# Move cur to new cur
cur = (cur.next).next
return dummyhead.next
Complexity
- Time complexity: O(n)
- Space complexity: O(1)
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