[Reverse Nodes in k-Group] LeetCode 25 #LinkedList

reverse.java

class Solution {
    public ListNode reverseKGroup(ListNode head, int k) {

        if (head == null) return null;
        
        ListNode start = head;
        ListNode end = head;
        
        for (int i = 0; i < k; i ++) {
            
            // if there node number is less than k.
            if (end == null) {
                return head;
            }
            end = end.next;
        }
        
        ListNode newHead = reverse(start, end);
        
        start.next = reverseKGroup(end, k);
        
        return newHead;
    }
    
    public ListNode reverse(ListNode a, ListNode b) {
        
        ListNode pre = null;
        ListNode cur = a;
        ListNode next = a;
        
        while(cur != b) {
            
            next = cur.next;
            cur.next = pre;
            pre = cur;          // 更新指针位置
            cur = next;
        }
        
       // 返回反转后的头结点
        return pre;
    }
}