025.md

25. Reverse Nodes in k-Group

Solution 1 (O(n))

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode reverseKGroup(ListNode head, int k) {
        if (k == 1)
            return head;
        ListNode dummy = new ListNode(-1);
        dummy.next = head;
        ListNode p = dummy;
        while(p != null)
            p = reverseNextK(p, k);
        return dummy.next;
    }

    public ListNode reverseNextK(ListNode head, int k) {
        ListNode p1 = head.next;
        for (int i = 0; i < k; i++) {
            if (p1 == null)
                return null;
            p1 = p1.next;
        }
        p1 = head.next;
        ListNode p2 = p1.next;
        ListNode ret = head.next;
        for (int i = 0; i < k - 1; i++) {
            ListNode p3 = p2.next;
            p2.next = p1;
            p1 = p2;
            p2 = p3;
        }
        head.next = p1;
        ret.next = p2;
        return ret;
    }
}