92. Reverse Linked List II

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LeetCode

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Problem

Reverse a linked list from position m to n. Do it in one-pass.

Note: 1 ≤ m ≤ n ≤ length of list.

Example:

Input: 1->2->3->4->5->NULL, m = 2, n = 4
Output: 1->4->3->2->5->NULL

Code

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode reverseBetween(ListNode head, int m, int n) {
        if(head == null) return head;

        ListNode dummy = new ListNode(-1);
        dummy.next = head;
        ListNode prev = dummy;
        // 1. 找到翻转起始点
        for(int i = 1; i < m; i++){
            prev = prev.next;
        }

        ListNode curr = prev.next;
        // 2. 翻转
        // 重点在prev并不移动
        for(int i = m; i < n; i++){
            ListNode next = curr.next;
            curr.next = next.next;
            next.next = prev.next;
            prev.next = next;
        }
        return dummy.next;
    }
}