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Reverse a linked list.
For linked list 1->2->3, the reversed linked list is 3->2->1
分析：
典型的3 pointers 问题。
1 /** 2 * Definition for ListNode. 3 * public class ListNode { 4 * int val; 5 * ListNode next; 6 * ListNode(int val) { 7 * this.val = val; 8 * this.next = null; 9 * } 10 * } 11 */ 12 public class Solution { 13 /** 14 * @param head: The head of linked list. 15 * @return: The new head of reversed linked list. 16 */ 17 public ListNode reverse(ListNode head) { 18 if (head == null) return head; 19 ListNode prev = null; 20 ListNode cur = head; 21 ListNode next = null; 22 while (cur != null) { 23 next = cur.next; 24 cur.next = prev; 25 prev = cur; 26 cur = next; 27 } 28 return prev; 29 } 30 }
Reverse a linked list from position m to n.
Note: Given m, n satisfy the following condition: 1 ≤ m ≤ n ≤ length of list.
Given 1->2->3->4->5->NULL, m = 2 and n = 4, return 1->4->3->2->5->NULL.
分析：
我们需要把list分成三段。首先得到第一段的最后一个node，然后reverse中间部分。最后把三个部分链接起来。
1 /** 2 * Definition for ListNode 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 /** 14 * @param ListNode head is the head of the linked list 15 * @oaram m and n 16 * @return: The head of the reversed ListNode 17 */ 18 public ListNode reverseBetween(ListNode head, int m , int n) { 19 if (m == n) return head; 20 ListNode tempHead = new ListNode(1); 21 tempHead.next = head; 22 23 // reach the end of the first part 24 ListNode partEnd = tempHead; 25 for(int k = 1; k < m; k++) { 26 partEnd = partEnd.next; 27 } 28 // save it later to connect to the last part. 29 ListNode secondPartTail = partEnd.next; 30 31 // reverse the middle part 32 ListNode prev = null; 33 ListNode current = partEnd.next; 34 ListNode next = null; 35 36 for (int p = 1; p <= n - m + 1; p++) { 37 next = current.next; 38 current.next = prev; 39 prev = current; 40 current = next; 41 } 42 // connect three parts 43 partEnd.next = prev; 44 secondPartTail.next = current; 45 46 return tempHead.next; 47 } 48 }
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原文地址：http://www.cnblogs.com/beiyeqingteng/p/5636524.html
