Swap Nodes in Pairs

LintCode-451.Swap Nodes in Pairs

Given a linked list, swap every two adjacent nodes and return its head.

Example: Given 1->2->3->4, you should return the list as 2->1->4->3.

Challenge: Your algorithm should use only constant space. You may not modify the values in the list, only nodes itself can be changed.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    /**
     * @param head a ListNode
     * @return a ListNode
     */
    public ListNode swapPairs(ListNode head) {
        if (head == null) {
            return head;
        }
        
        ListNode dummy = new ListNode(-1);
        dummy.next = head;
        
        ListNode current = dummy;
        
        while (current.next != null && current.next.next != null) {
            ListNode first = current.next;
            ListNode second = current.next.next;
            
            first.next = second.next;
            second.next = first;
            current.next = second;
            
            current = first;
        }
        
        return dummy.next;
    }
}

Hope this helps,
Michael

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