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Reverse Nodes in k-Group

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Description

https://leetcode.com/problems/reverse-nodes-in-k-group/description/

Solution

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/**
 * 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 (head == null || head.next == null) return head;
        ListNode new_head = new ListNode(-1);
        new_head.next = head;
        ListNode traverse = new_head;
        while (true) {
            if (testReverse(traverse, k) == true) {
                traverse = reverseList(traverse, k);   
            }
            else return new_head.next;
        }
    }
    
    //Return the head tail of partial list
    public ListNode reverseList(ListNode start, int k) {
        ListNode pre = start.next;
        ListNode current = start.next.next;
        for (int i = 0; i < k - 1; i++) {
            ListNode temp = current.next;
            current.next = pre;
            pre = current;
            current = temp;            
        }
        ListNode originNext = start.next;
        start.next = pre;
        originNext.next = current;
        return originNext;
    }
    
    public boolean testReverse(ListNode start, int k) {
        while(k > 0) {
            if (start.next == null) return false;
            k -= 1;
            start = start.next;
        }
        return true;
    }
}