[leetcode]143. Reorder List重排链表

Given a singly linked list L: L0→L1→…→Ln-1→Ln,
reorder it to: L0→Ln→L1→Ln-1→L2→Ln-2→…

You may not modify the values in the list's nodes, only nodes itself may be changed.

思路：

1.  find mid

2.  cut list 

3.  reverse slow part

4.  merge two parts 

代码：

class Solution {
    public void reorderList(ListNode head) {
        // corner case
        if(head == null || head.next == null) return;
        // init 
        ListNode fast = head;
        ListNode slow = head;
        ListNode prev = null; 
        // find mid
        while(fast != null && fast.next != null){
            prev = slow;
            slow = slow.next;
            fast = fast.next.next;
        }
        //cut list
        prev.next = null;
        //recurse  slow part
        slow = reverse(slow);
        fast = head;
        // merge two parts
        while(fast.next!=null){
            ListNode temp = fast.next; 
            fast.next = slow; 
            slow = slow.next;
            fast.next.next = temp;
            fast = temp;
        } 
        fast.next = slow; 
    }
    
    private ListNode reverse(ListNode head){
        ListNode cur = head;
        ListNode pre = null;
        while(cur != null){
            ListNode temp = cur.next;
            cur.next = pre; 
            pre=cur;
            cur=temp;
        }
        return pre; 
    }
}