ReverseNodesInKGroup.java

public class Solution {
    public ListNode reverseKGroup(ListNode head, int k) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(k == 0 || k == 1) return head;
        ListNode cur = head;
        int length = 0;
        while (cur != null){
            cur = cur.next;
            length++;
        }
        int multi = length / k;
        if(multi == 0) return head;
        ListNode preTail = null, curHead = null, curTail = null;
        ListNode preNode = null, nextNode = null;
        cur = head;
        for(int j = 0; j < multi; j++) {
            preNode = null;
            for(int i = 0; i < k; i++) {
                if(cur != null) {
                    nextNode = cur.next;
                    cur.next = preNode;
                    preNode = cur;
                }
                if(i == 0) curTail = cur;
                if(i == (k - 1)) curHead = cur;
                cur = nextNode;
            }
            if(preTail == null) head = curHead;
            else preTail.next = curHead;
            preTail = curTail;
        }
        curTail.next = cur;
        return head;
    }
}