linked-list
pointers
recursion
Given the head of a singly linked list, swap every two adjacent nodes and return the head of the modified list. You must solve it by only changing node links (not the values stored inside the nodes).
Input / output
- Input:
head: ListNode(JSON test fixture is a plain array of node values, e.g.[1, 2, 3, 4]means1 -> 2 -> 3 -> 4 -> null) - Output:
ListNode(serialized the same way)
Constraints
- The number of nodes in the list is between 0 and 100.
0 <= node value <= 100
Follow-up
Can you solve it both iteratively (with a dummy head and a prev pointer) and recursively, and explain why the recursive version costs O(n) call-stack space that the iterative version avoids?
Examples
Example 1
Input: head = [1,2,3,4]
Output: [2,1,4,3]
Example 2 (empty list)
Input: head = []
Output: []
Example 3 (single node)
Input: head = [1]
Output: [1]
🔒 6 hidden
Running will execute all 9 cases, including 6 hidden ones.