143.重排链表

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public void reorderList(ListNode head) {
        //原链表的左半段+反转后的右半端合并后的结果
        if(head == null){
            return;
        }
        ListNode mid = middleNode(head);
        ListNode l1 = head;
        ListNode l2 = mid.next;
        mid.next = null;
        l2 = reverseList(l2);
        mergeList(l1,l2);


    }
    //找到中间节点
    public ListNode middleNode(ListNode head){
        ListNode slow = head;
        ListNode fast = head;
        while(fast.next != null && fast.next.next != null){
            slow = slow.next;
            fast = fast.next.next;
        }
        return slow;
    }

    //反转链表
    public ListNode reverseList(ListNode head){
        ListNode pre = null;
        ListNode cur = head;
        while(cur!= null){
            ListNode next = cur.next;
            cur.next = pre;
            pre = cur;
            cur = next;
        }
        return pre;
    }


    //合并链表
    public void mergeList(ListNode L1,ListNode L2){
        ListNode l1_tmp;
        ListNode l2_tmp;
        while(L1!=null && L2 != null){
            l1_tmp = L1.next;
            l2_tmp = L2.next;

            L1.next = L2;
            L1 = l1_tmp;

            L2.next = L1;
            L2 = l2_tmp;
        }
    }

}