143Reorder List
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题目链接:https://leetcode.com/problems/reorder-list/
题目:
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}.
解题思路:
第一种思路:
1. 第一次遍历链表,将链表结点入栈
2. 第二次遍历链表,同时将栈中结点弹出,插入到当前遍历到的结点后面。例如,遍历第一个结点,弹出最后一个结点,将弹出的结点链接到第一个结点之后。
3. 终止条件为,当前遍历的结点的后继是弹出的结点(偶数个结点的情况),或者当前遍历的结点就是弹出的结点(奇数个结点的情况)
4. 这种方式需要O(n)的空间,运行也比较慢
第二种思路:
将该题拆分为三个基本操作:
1. 将链表从中间断成两半,方法是 walker 和 runner,当 runner 到达链表终点时,walker 到达链表中间。
2. 将后半段链表进行就地转置操作
3. 将前后两段链表归并成一个完整链表
第二种思路更加简洁,可以参考大神的做法:http://blog.csdn.net/linhuanmars/article/details/21503215
代码实现:
第一种:
/** * Definition for singly-linked list. * public class ListNode { * int val; * ListNode next; * ListNode(int x) { val = x; } * } */public class Solution { public void reorderList(ListNode head) { if(head == null) return; LinkedList<ListNode> stack = new LinkedList(); ListNode p = head; int len = 0; while(p != null) { len ++; stack.push(p); p = p.next; } ListNode q = head; p = stack.pop(); while(q.next != p && q != p) { ListNode tmp = q.next; q.next = p; p.next = tmp; q = q.next.next; p = stack.pop(); } p.next = null; }}
13 / 13 test cases passed.Status: AcceptedRuntime: 9 ms
第二种:
/** * Definition for singly-linked list. * public class ListNode { * int val; * ListNode next; * ListNode(int x) { val = x; } * } */public class Solution { public void reorderList(ListNode head) { if(head == null || head.next == null || head.next.next == null) return; ListNode p = head.next; ListNode q = head.next.next; while(q != null && q.next != null && q.next.next != null) { p = p.next; q = q.next.next; } ListNode head2 = p.next; q = head2; while(q.next != null) { ListNode tmp = q.next; q.next = tmp.next; tmp.next = head2; head2 = tmp; } p = head; q = head2; ListNode h = new ListNode(0); while(p != null && q != null) { ListNode tmp1 = p.next; ListNode tmp2 = q.next; h.next = p; p.next = q; p = tmp1; q = tmp2; h = h.next.next; } if(p != null) { h.next = p; p.next = null; } }}
13 / 13 test cases passed.Status: AcceptedRuntime: 3 ms
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