[LeetCode] 24. 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.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode swapPairs(ListNode head) {
        if(null == head)
            return null;
        
        ListNode resPreHead = new ListNode(0);
        ListNode resIndex = resPreHead;
        
        ListNode preHead = new ListNode(0);
        preHead.next = head;
        ListNode index = preHead;
        
        while(null != index.next){
            ListNode a = index.next;
            ListNode b = null;
            if(null != a)
                b = a.next;
            if(null == b){
                index.next = null;
            }else{
                index.next = b.next;
            }
                
            if(null != b){
                resIndex.next = b;
                resIndex = b;
                resIndex.next = null;
            }
            
            if(null != a){
                resIndex.next = a;
                resIndex = a;
                resIndex.next = null;
            }
        }
        
        return resPreHead.next;
    }
}