92.反转链表II

/*** Definition for singly-linked list.* public class ListNode {*     int val;*     ListNode next;*     ListNode() {}*     ListNode(int val) { this.val = val; }*     ListNode(int val, ListNode next) { this.val = val; this.next = next; }* }*/
class Solution {public ListNode reverseBetween(ListNode head, int left, int right) {ListNode dummy = new ListNode(0);dummy.next = head;ListNode pre = dummy;// 第 1 步：从虚拟头节点走 left - 1 步，来到 left 节点的前一个节点for(int i = 0;i<left-1;i++){pre = pre.next;}// 第 2 步：从 pre 再走 right - left + 1 步，来到 right 节点ListNode rightNode = pre;for(int i = 0;i<right - left + 1;i++){rightNode = rightNode.next;}// 第 3 步：切断出一个子链表（截取链表）ListNode leftNode = pre.next;ListNode cur = rightNode.next;pre.next = null;rightNode.next = null;// 第 4 步：反转链表的子区间reverseLinkedList(leftNode);// 第 5 步：接回到原来的链表中pre.next = rightNode;leftNode.next = cur;return dummy.next;}public void reverseLinkedList(ListNode head){ListNode pre = null;ListNode cur = head;while(cur != null){ListNode next = cur.next;cur.next = pre;pre = cur;cur = next;}}
}