Reverse Nodes in k-Group

Given a linked list, reverse the nodes of a linked list k at a time and return its modified list.

If the number of nodes is not a multiple of k then left-out nodes in the end should remain as it is.

You may not alter the values in the nodes, only nodes itself may be changed.

Only constant memory is allowed.

For example,
Given this linked list: 1->2->3->4->5

For k = 2, you should return: 2->1->4->3->5

For k = 3, you should return: 3->2->1->4->5

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode reverseKGroup(ListNode head, int k) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(k<=1 || head==null || head.next==null) return head;
        
        ListNode prev = new ListNode(0);
        prev.next = head;
        head = prev;
        
        ListNode cur=prev.next;
        while(cur!=null){
            int counter = k;
            while(cur!=null && counter>1){
                cur = cur.next;
                counter--;
            }
            
            if(cur!=null){
                cur=prev.next;
                counter=k;
                while(counter>1){
                    ListNode temp = cur.next;
                    cur.next=temp.next;
                    temp.next=prev.next;
                    prev.next=temp;
                    counter--;
                }
                prev = cur;
                cur = prev.next;
            }
        }
        
        return head.next;
    }
}

 Second Round:

Divide the original linked list into groups, while each group contains k elements. reverse this sublist. ---> do it as reverse linked list.

It will be much easier to understand!!!

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode reverseKGroup(ListNode head, int k) {
        if(head == null) return null;
        ListNode header = new ListNode(-1);
        header.next = head;
        ListNode curTail = head, curHead = head, perTail = header;
        while(curHead != null){
            for(int i = 1; i < k; i ++){
                curTail = curTail.next;
                if(curTail == null){
                    perTail.next = curHead;
                    return header.next;
                }
            }
            ListNode nextHead = curTail.next;
            curTail.next = null;
            ListNode newHead = reverseList(curHead);
            perTail.next = newHead;
            perTail = curHead;
            curHead = nextHead;
            curTail = nextHead;
        }
        return header.next;
    }
    
    public ListNode reverseList(ListNode head){
        ListNode per = null;
        ListNode cur = head;
        while(cur != null){
            ListNode tmp = cur.next;
            cur.next = per;
            per = cur;
            cur = tmp;
        }
        return per;
    }
}

 第三遍：

public class Solution {
    public ListNode reverseKGroup(ListNode head, int k) {
        ListNode header = new ListNode(-1);
        header.next = head;
        ListNode cur = header, fast = header;
        while(fast != null){
            for(int i = 0; i < k; i ++){
                fast = fast.next;
                if(fast == null) return header.next;
            }
            ListNode next = fast.next;
            fast.next = null;
            reverseList(cur.next);
            ListNode newCur = cur.next;
            cur.next.next = next;
            cur.next = fast;
            cur = newCur;
            fast = newCur;
        }
        return header.next;
    }
    
    public ListNode reverseList(ListNode head){
        ListNode cur = head, pre = head.next;
        while(pre != null){
            ListNode tmp = pre.next;
            pre.next = cur;
            cur = pre;
            pre = tmp;
        }
        return cur;
    }
}