24. Exchange the nodes in the linked list (linked list)

subject ：

Given a linked list , Two or two exchange the adjacent nodes , And return the list after exchange .

You can't just change the values inside the node , It needs to actually exchange nodes .

public class Liangliangjiaohuanlianbiaozhongdejiedian {
    static class ListNode{
        int val;// data ： Node data 
        ListNode next;// object ： Object of node 

        ListNode(int val){
            this.val = val;
        }
    }

    public ListNode swapPairs(ListNode head){
            // Set up a virtual head node 
            ListNode dummy = new ListNode(0);
            dummy.next = head;
            // Define the current node 
            ListNode cur = dummy;
            while (cur.next!=null && cur.next.next!=null){
                // Define temporary nodes 
                ListNode temp = cur.next;
                ListNode temp1= cur.next.next.next;

                // Step one 
                cur.next = cur.next.next;
                // Step two 
                cur.next.next = temp;
                // Step three 
                cur.next.next.next = temp1;

                //cur Move two bits for the next round of exchange 
                cur = cur.next.next;
            }
            return dummy.next;
    }
    // Printout 
    static void print(ListNode listNode){
        // Create a linked list node 
        while (listNode!=null){
            if(listNode.next==null){
                System.out.print(listNode.val);
            }else {
                System.out.print(listNode.val + "->");
            }
            listNode = listNode.next;
        }
    }
    public static void main(String[] args) {
        // Create the first node 
        ListNode nodeSta = new ListNode(1);
        // Declare a variable to represent the current node during the move 
        ListNode nextnode;
        nextnode = nodeSta;

        for (int i = 2; i <=6 ; i++) {
            // Generate new nodes 
            ListNode node = new ListNode(i);
            // The mobile node points to the new node ( Connect the new node 
            nextnode.next = node;
            // Move the current node back 
            nextnode = nextnode.next;
        }
        // Linked list generation completed 
        // Print directly from the first node 
        print(nodeSta);
        System.out.println();
        Liangliangjiaohuanlianbiaozhongdejiedian liangliangjiaohuanlianbiaozhongdejiedian = new Liangliangjiaohuanlianbiaozhongdejiedian();
        ListNode res = liangliangjiaohuanlianbiaozhongdejiedian.swapPairs(nodeSta);
        print(res);
    }
}

Time complexity ：O（n）

Spatial complexity ：O（1）