[leetcode]Reorder List

新博文地址：[leetcode]Reorder List

Reorder List

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

You must do this in-place without altering the nodes' values.

For example,
Given {1,2,3,4}, reorder it to {1,4,2,3}.

 

算法思路：

设置快慢指针将前半段与后半段分开，然后将后半段逆序，再逐个插入前半段，时间复杂度O(n)，空间复杂度不定

思路1：

后半段的逆序，设置三指针，在原list基础上逆序，空间复杂度O(1)

后面还有一些题会用这个思路，这里就不实现了。

 

代码如下：

 

public class Solution {
    public void reorderList(ListNode head) {
        if(head == null || head.next == null) return ;
        ListNode hhead = new ListNode(0);
        hhead.next = head;
        ListNode fast = hhead;
        ListNode slow = hhead;
        while(fast != null && fast.next != null){
            fast = fast.next.next;
            slow = slow.next;
        }
        ListNode stackPart = slow.next;
        slow.next = null;
        Stack<ListNode> stack = new Stack<ListNode>();
        while(stackPart != null){
            stack.push(stackPart);
            stackPart = stackPart.next;
        }
        ListNode insert = head;
        while(!stack.isEmpty()){
            ListNode tem = stack.pop();
            tem.next = insert.next;
            insert.next = tem;
            insert = tem.next;
        }
    }
}