23. Swap Nodes in Pairs

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.

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Solution1: Interative

/**
 * 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) {
        
        if(head == null || head.next == null) 
            return head;
    
        ListNode prev=null, p1 = head, p2 = head.next, next = p2.next;
        head = p2; // new head
    
        while(true){
            if(prev != null) 
                prev.next = p2;
            p2.next = p1;
            p1.next = next;
    
            // update
            prev = p1;
            p1 = next;
            if(p1 == null) 
                return head; // end
            p2 = p1.next;
            if(p2 == null) 
                return head; // end
            next = p2.next;
        }
    }
}

 

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Solution2: Recursive

/**
 * 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) {
        
        if(head == null)        return null;
        if (head.next == null)  return head;
        
        ListNode nextList = swapPairs(head.next.next);
        ListNode p2 = head.next;
        head.next = nextList;
        p2.next = head;
        return p2; // return the new head

    }
}