Swap Nodes in Pairs

Problem

Given the head of a linked list, swap every two adjacent nodes and return the new head. Must swap nodes, not just values.

Constraints

• 0 ≤ length ≤ 100

Approach — Linked List

This is a Linked List problem. The idea: rework the list with careful pointer manipulation — often a dummy head and fast/slow pointers — in a single pass. Work through the reference code below line by line, then re-derive it yourself in the editor — that's how the pattern sticks.

Complexity: O(n) time, O(1) space.

Solution code

Python

class Solution:
    def swapPairs(self, head):
        dummy = ListNode(0, head)
        prev = dummy
        while prev.next and prev.next.next:
            a = prev.next
            b = a.next
            a.next = b.next
            b.next = a
            prev.next = b
            prev = a
        return dummy.next

Java

class Solution {
    public ListNode swapPairs(ListNode head) {
        if (head == null || head.next == null) return head;
        ListNode second = head.next;
        head.next = swapPairs(second.next);
        second.next = head;
        return second;
    }
}

Practice it

Reading a solution isn't the same as being able to write it under pressure. Open this problem in the in-browser editor, hide the solution, and solve it from scratch — your code runs against real test cases instantly.