26 Dec 24. Swap Nodes in Pairs
Given 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]
Constraints:
- The number of nodes in the list is in the range
[0, 100]. 0 <= Node.val <= 100
Visualization
Complexity
- Time: O(n)
- Space: O(n)
# 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]:
if head is None or head.next is None:
return head
result = head.next
head.next = self.swapPairs(result.next)
result.next = head
return result
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