Swap Nodes in Pairs

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

答案
recursion答案，很简洁，可惜只要recursion就不是O(1) space了
思路是swap current pairs, recursively swap later pairs

class Solution {
    public ListNode swapPairs(ListNode head) {
        if(head != null && head.next != null) {
            ListNode next = head.next;
            ListNode next2 = next.next;
            next.next = head;
            head.next = swapPairs(next2);
            return next;
        }
        return head;
    }
}

public class Solution {
    public static ListNode swapPairs(ListNode head) {
        if (head == null || head.next == null) return head;
        ListNode first    = head;
        ListNode second   = head.next;
        ListNode previous = null, ret = second;
        while(first != null && second != null) {
            // swap first and second
            first.next = second.next;
            second.next = first;
            if(previous != null) previous.next = second;
            previous = first;
            
            first = first.next;
            if(first != null) second = first.next;
        }
        return ret;        
    }
}