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

swapPairs.java

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

For example,
Given 1->2->3->4, you should return the list as 2->1->4->3.

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;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode swapPairs(ListNode head) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(head == null) return null;
        
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        
        ListNode cur = head;
        ListNode ahead = null;
        if(head.next != null) {
            ahead = head.next;
        } else {
            return cur;
        }
        
        ListNode prev = dummy;
        while(ahead != null) {
            prev.next = swap(cur, ahead);
            prev = cur;
            cur = cur.next;
            if(cur != null)
                ahead = cur.next;
            else
                ahead = null;
        }
        return dummy.next;
    }
    
    public ListNode swap(ListNode left, ListNode right) {
        left.next = right.next;
        right.next = left;
        return right;
    }
}