# Swap Nodes in Pairs

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

**Example:**

```
Given 1->2->3->4, you should return the list as 2->1->4->3.
```

**Note:**

* Your algorithm should use only constant extra space.
* You may **not** modify the values in the list's nodes, only nodes itself may be changed.

在翻转特定区域的linkedlist时，往往受影响的还包括要反转的第一个节点的前面的一个节点，因此考虑\
使用了dummy node

```java
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode swapPairs(ListNode head) {
        if(head == null || head.next == null){
            return head;
        }
        
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode run = dummy;
        while(run != null && run.next != null){
            helper(run);
            run = run.next.next;
        }
        
        return dummy.next;
    }
    
    public void helper(ListNode head){
        if(head.next.next == null) return;
        ListNode cur = head.next;
        head.next = cur.next;
        cur.next = head.next.next;
        head.next.next = cur;
        
    }
}
```


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