LeetCode143. 重排链表

class Solution {
    public void reorderList(ListNode head) {
        //先找到链表中点，分成左右两部分
        //将右部分逆序，再对两部分进行顺序合并
        if(head==null || head.next == null)
            return;
        ListNode mid = findMid(head);
        ListNode l2 = mid.next;
        mid.next = null;
        l2 = reverse(l2);
        mergeList(head, l2);
    }
    //链表合并
    public void mergeList(ListNode l1,ListNode l2)
    {
        //将l2的节点插入l1的两两节点之中
        ListNode l1_next = null;
        ListNode l2_next = null;
        while(l2 != null)
        {
            l1_next = l1.next;
            l2_next = l2.next;
            l1.next = l2;
            l2.next = l1_next;
            l1 = l1_next;
            l2 = l2_next;
        }
    }
    //链表逆序
    public ListNode reverse(ListNode node)
    {
        ListNode next = null;
        ListNode pre = null;
        while(node!=null)
        {
            next = node.next;
            node.next = pre;
            pre = node;
            node = next;
        }
        return pre;
    }
    //找链表终点
    public ListNode findMid(ListNode node)
    {
        ListNode fast = node;
        ListNode slow = node;
        do
        {
            fast = fast.next.next;
            slow = slow.next;
        }while(fast != null && fast.next != null);
        return slow;
    }
}