leetcode 24

public class Solution {
    public static void main(String[] args) {
        ListNode head = new ListNode();
        head.val=1;
        head.next = null;
        for (int i = 1; i < 6; i++) {
            ListNode node = new ListNode(i,null);
            node.next = head.next;
            head.next = node;
        }
//        ListNode head = null;
        Show.show(head);
        head = swapPairs(head);
        Show.show(head);
    }
    public static ListNode swapPairs(ListNode head) {
        //leetcode 24  https://leetcode-cn.com/problems/swap-nodes-in-pairs/
        if(head==null||head.next==null){
            return head;
        }

        ListNode front,behind;
        front = head.next;
        behind = head;
        int i=1;
        ListNode listHead = new ListNode();
        ListNode record = null;
        while(front!=null){
            if(i==1){
                listHead = front;
                behind.next = front.next;
                front.next = behind;
            }
            else{
                record.next = front;
                behind.next = front.next;
                front.next = behind;
            }
            record = behind;
            if(behind.next!=null){
                front = behind.next.next;
            }
            else{
                front = null;
            }
            behind = behind.next;
            i++;
        }
        return listHead;
    }
}