swapAdjacentNodes.java

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

NOTE: Your algorithm should use only constant space. You may not modify the values in the list; only nodes themselves can be changed.



Problem Constraints
 1 <= |A| <= 106 



Input Format
 The first and the only argument of input contains a pointer to the head of the given linked list. 



Output Format
 Return a pointer to the head of the modified linked list. 



Example Input
 Input 1: 

 A = 1 -> 2 -> 3 -> 4
 Input 2: 

 A = 7 -> 2 -> 1


Example Output
 Output 1: 

 2 -> 1 -> 4 -> 3
 Output 2: 

 2 -> 7 -> 1


Example Explanation
Explanation 1:

 In the first example (1, 2) and (3, 4) are the adjacent nodes. Swapping them will result in 2 -> 1 -> 4 -> 3
Explanation 2:

 In the second example, 3rd element i.e. 1 does not have an adjacent node, so it won't be swapped. 
*/


//PROGRAM:
/**
 * Definition for singly-linked list.
 * class ListNode {
 *     public int val;
 *     public ListNode next;
 *     ListNode(int x) { val = x; next = null; }
 * }
 */
public class Solution {
    public ListNode swapPairs(ListNode A) {
        ListNode dummy=new ListNode(0);
        dummy.next=A;
        ListNode prev=dummy;
        
        while(prev.next!= null && prev.next.next!=  null){
            ListNode first=prev.next;
            ListNode second=prev.next.next;
            
            first.next=second.next;
            second.next=first;
            prev.next=second;
            
            prev=first;
        }
        return dummy.next;
    }
}