24. Swap Nodes in Pairs.JAVA

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode swapPairs(ListNode head) {
        if(head == null || head.next == null){
            return head;
        }
     ListNode curr = head.next.next;
     ListNode prev = head;
     head = head.next;
     head.next = prev;
     while(curr != null && curr.next != null){
         prev.next = curr.next;
         prev = curr;
         ListNode temp = curr.next.next;
         curr.next.next = curr;
         curr = temp;
     }
     prev.next = curr;
     return head;
    }
}