Reverse Nodes in k-Group

ID: 450; hard; K组翻转链表

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Solution 1 (Java)

/**
 * Definition for ListNode
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */

public class Solution {
    /**
     * @param head: a ListNode
     * @param k: An integer
     * @return: a ListNode
     */
    public ListNode reverseKGroup(ListNode head, int k) {
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode cur = dummy;

        while (true) {
            cur = reverseBetween(cur, k);
            if (cur == null) break;
        }
        return dummy.next;
    }


    // modified from Reverse Linked List II
    public ListNode reverseBetween(ListNode head, int k) {
        if (head == null) return head;

        // 1. find the k-th node 
        ListNode kNode = head;
        for (int i = 0; i < k; i++) {
            kNode = kNode.next;
            if (kNode == null) return null;
        }

        ListNode firstNode = head.next;
        ListNode kNextNode = kNode.next;

        // 2. reverse
        ListNode prev = null, next;
        ListNode curr = firstNode;
        while (curr != kNextNode) {
            next = curr.next;
            curr.next = prev;
            prev = curr;
            curr = next;
        }

        // 3. connect the lists
        head.next = kNode;
        firstNode.next = kNextNode;
        return firstNode;
    }
}

Notes

• We use a similar approach used in Reverse Linked List II. Using the helper function reverseBetween, we reverse k nodes each time and return the end of of the linked list for each k group. If the end node is null, we know that the remaining nodes cannot constitute a k group.

• Inside the helper function, note that the head is always one node before the starting node that is to be reversed. Then, we keep track of the first node that is to be reversed (firstNode) and the node after k-th node (kNextNode) because we need to connect the reversed linked list eventually with a start and an end.