[LeetCode] 25.Reverse Nodes in k-Group-2

　　从这位大佬这里抄了解法2：https://www.cnblogs.com/grandyang/p/4441324.html

　　

解法二：

class Solution {
public:
    ListNode* reverseKGroup(ListNode* head, int k) {
        ListNode *dummy = new ListNode(-1), *pre = dummy, *cur = pre;
        dummy->next = head;
        int num = 0;
        while (cur = cur->next) ++num;
        while (num >= k) {
            cur = pre->next;
            for (int i = 1; i < k; ++i) {
                ListNode *t = cur->next;
                cur->next = t->next;
                t->next = pre->next;
                pre->next = t;
            }
            pre = cur;
            num -= k;
        }
        return dummy->next;
    }
};

我们也可以使用递归来做，我们用 head 记录每段的开始位置，cur 记录结束位置的下一个节点，然后我们调用 reverse 函数来将这段翻转，然后得到一个 new_head，原来的 head 就变成了末尾，这时候后面接上递归调用下一段得到的新节点，返回 new_head 即可，参见代码如下：

　　总结出错的原因，1.没有从listnode的length和k的长度关系来考虑，把问题变得很复杂。2.妄图手动修改listnode的head tail两个节点的四条指针，导致问题复杂化，应该用for循环和tempNode移位。

　　抄的代码运行结果：

　　

Success

Details 

Runtime: 56 ms, faster than 55.78% of Python3 online submissions for Reverse Nodes in k-Group.

Memory Usage: 14.6 MB, less than 5.88% of Python3 online submissions for Reverse Nodes in k-Group.

 

Submission Detail

81 / 81 test cases passed.	Status:  Accepted
Runtime: 56 ms Memory Usage: 14.6 MB	Submitted: 0 minutes ago

class Solution:
    def reverseKGroup(self, head: ListNode, k: int) -> ListNode:
        # 0 1
        if head == None or head.next == None:
            return head
        # k == 0
        if k == 0 or k == 1:
            return head
        # swap node
        dummy = ListNode(0)
        dummy.next = head
        prevNode = dummy
        currentNode = dummy
        length = 0  #length of head
        while currentNode.next != None :
            length = length +1
            currentNode = currentNode.next
        while(length >= k): #listnode is longer than k
            currentNode = prevNode.next
            for index in range(1,k):
                tempNode = currentNode.next
                currentNode.next = tempNode.next
                tempNode.next = prevNode.next
                prevNode.next = tempNode
            head = dummy.next   #headlist will lose first node
            prevNode = currentNode
            length -= k
        return head

　　40ms:

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def reverseKGroup(self, head, k):
        cur = head
        count = 0
        while cur and count < k:
            cur = cur.next
            count += 1
            
        # cur = 4 
        
        if count == k:
            last = self.reverseKGroup(cur, k)
            while count > 0:
                next_node = head.next
                head.next = last
                last = head
                head = next_node
                count -= 1
            head = last
        return head