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leetcode 92. reverse linked list ii

admin 11月 13, 2020 0

92. Reverse Linked List II

Difficulty:: Medium

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

*Note: *1 ≤ m ≤ n ≤ length of list.

Example:

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

Solution

Language: Java


 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class  {
    public ListNode reverseBetween(ListNode head, int m, int n) {
        if (head == null || m <= 0 || n <= 0 || m >= n) {
            return head;
        }
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode prev = dummy;
        ListNode cur = head;
        for (int i = 0; i < m - 1; i++) {
            prev = cur;
            cur = cur.next;
        }
        ListNode start = prev;
        ListNode end = cur;
        for (int i = 0; i < (n - m + 1); i++) {
            ListNode next = cur.next;
            cur.next = prev;
            prev = cur;
            cur = next;
        }
        start.next = prev;
        end.next = cur;
        return dummy.next;
    }
}