24. Swap Nodes in Pairs
mediumSwap every two adjacent nodes in a singly linked list — without altering node values. A miniature version of Reverse Nodes in k-Group with k=2, perfect for drilling the dummy-head plus three-pointer rewire pattern.
By Alex Chen, Founder, InterviewChamp.AI · Last verified
Problem
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).
Constraints
The number of nodes in the list is in the range [0, 100].0 <= Node.val <= 100
Examples
Example 1
head = [1,2,3,4][2,1,4,3]Example 2
head = [][]Example 3
head = [1][1]Solve it now
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Hints
Progressive — try the first before opening the next.
Hint 1
A dummy head simplifies the very first swap, which would otherwise need to change the head pointer.
Hint 2
For each pair, you need three pointers: the node before the pair, the first node of the pair, the second node of the pair. Rewire them all in one fixed sequence.
Hint 3
After swapping a pair, advance your 'previous' pointer to the new tail of that pair so the next iteration handles the next two nodes cleanly.
Solution approach
Reveal approach
Dummy-head with three-pointer rewiring. Allocate a dummy node and link dummy.next = head. Use prev = dummy. Loop while prev.next and prev.next.next are both non-null: let first = prev.next and second = prev.next.next. Rewire in fixed order: first.next = second.next; second.next = first; prev.next = second. Now the pair is swapped and first is the tail of this swapped pair. Advance prev = first and continue. Return dummy.next. Recursive variant: pair up head and head.next, recurse on head.next.next, then rewire. O(n) time, O(1) extra space iteratively (O(n) recursion stack for the recursive version).
Complexity
- Time
- O(n)
- Space
- O(1)
Related patterns
- linked-list
- dummy-head
- pointer-rewiring
- in-place-linked-list-reversal
Related problems
Asked at
Companies reported asking this problem (sourced from public Glassdoor, Blind, and Levels.fyi interview posts).
- Amazon
- Microsoft
- Meta
- Bloomberg
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