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Leetcode: Reverse Linked List II

Reverse a linked list from position m to n. Do it in-place and in one-pass.For example:Given 1->2->3->4->5->NULL, m = 2 and n = 4,return 1->4->3->2->5->NULL.Note:Given m, n satisfy the following condition:1 ≤ m ≤ n ≤ length of list.

Analysis: 这道题还是想了我蛮久,先是题意理解错了,以为只是m和n这两个位置对调,结果发现其实是m到n之间的所有元素都需要调换。一时之间没有想到怎样做reverse比较好,参考了一下网上的思路,发现这样做比较好:还是要用Runner Technique,还是要用Dummy Node;两个指针: npointer指到n的位置,mpointer指到m的前一位;每一次把mpointer后一位的元素放到npointer的后一位:mpointer.next.next = npointer.next;直到mpointer.next = npointer为止(m与n重合)

Original linked list:       1->2->3->4->5->6->7; m = 3, n =6

Step1:        1->2->4->5->6->3->7    

Step2:      1->2->5->6->4->3->7           

......

Result:      1->2->6->5->4->3->7

Note that pointer m is switching to right one by one in each step, but pointer n remains no change.

再次体现了在链表题中使用Dummy Node, 并且对当前node.next进行操作的好处。

 

Notice: 像这种链表删除插入操作,在删除插入之前,最好先拷贝一下被删除节点的下一个节点,以及插入位置的下一个节点,把它们存在一个变量ListNode store里面,直接去访问这个变量,而不要用类似mpointer.next的方式存着他们,因为这样受制于mpointer,mpointer一旦变化,就找不到这些删除/插入节点的下一个节点了。

 1 /** 2  * Definition for singly-linked list. 3  * public class ListNode { 4  *     int val; 5  *     ListNode next; 6  *     ListNode(int x) { 7  *         val = x; 8  *         next = null; 9  *     }10  * }11  */12 public class Solution {13     public ListNode reverseBetween(ListNode head, int m, int n) {14         ListNode prev = new ListNode(-1);15         prev.next = head;16         ListNode mpointer = prev; //point to m-1 position17         ListNode npointer = prev; //point to n position18         while (m > 1) {19             mpointer = mpointer.next;20             m--;21         }22         while (n > 0) {23             npointer = npointer.next;24             n--;25         }26         while (mpointer.next != npointer) {27             ListNode mnext = mpointer.next.next;28             ListNode nnext = npointer.next;29             mpointer.next.next = nnext;30             npointer.next = mpointer.next;31             mpointer.next = mnext;32         }33         return prev.next;34     }35 }