Reverse Nodes in k-Group

public class Solution {
    public ListNode reverseKGroup(ListNode head, int k) {
        // Start typing your Java solution below
        // DO NOT write main() function
        
        if(head == null || k == 1)
            return head;
            
        int len = 0;
        ListNode p = head;
        while(p != null){
            p = p.next;
            len ++;
        }
        
        ListNode safeG = new ListNode(-1);
        safeG.next = head;
        ListNode pre = safeG, cur = head, post = head.next;
        
        int m = len / k;
        for(int i = 0; i < m; i++){
            post = cur.next;
            //reverse(pre, cur, post, k);
            int j = 0;            
            while(post != null){
                ListNode tmp = post.next;
                post.next = cur;
                cur = post;
                post = tmp;
                j ++;
                if(j == k - 1)
                    break;
            }
            ListNode tmp = pre.next;
            pre.next = cur;
            tmp.next = post;
            pre = tmp;
            cur = pre.next;
            
        }
        
        return safeG.next;
        
    }
    
    public void reverse(ListNode pre, ListNode cur, ListNode post, int k){
        int i = 0;            
        while(post != null){
            ListNode tmp = post.next;
            post.next = cur;
            cur = post;
            post = tmp;
            i ++;
            if(i == k - 1)
                break;
        }
    }
}