24. Swap Nodes in Pairs

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LeetCode

Approach 1: Iterative Approach, Time Complexity: O(N), where N is the size of the linked list

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/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode swapPairs(ListNode head) {
        ListNode dummy=new ListNode(-1),p=dummy;
        dummy.next=head;
        while(head!=null && head.next!=null){
            p.next=head.next;
            head.next=head.next.next;
            p.next.next=head;
            head=head.next;
            p=p.next.next;
        }
        return dummy.next;
    }
}

Approach 2: Recursive Approach, Time Complexity: O(N)

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/**
 * 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 firstNode=head;
        ListNode secondNode=head.next;
        
        firstNode.next=swapPairs(secondNode.next);
        secondNode.next=firstNode;
        
        return secondNode;
    }
}

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