题解 | #链表内指定区间反转#

# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None
#
# 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
#
# 
# @param head ListNode类 
# @param m int整型 
# @param n int整型 
# @return ListNode类
#
class Solution:
    def reverseBetween(self , head: ListNode, m: int, n: int) -> ListNode:
        # write code here
        if not head or not head.next or n==m:
            return head
        
        count = 1
        pre, cur, nex = None, head, head.next
        stack = []
        while nex:
            if m <= count < n:
                stack.append(cur)
                cur, nex = cur.next, nex.next
                count += 1
                continue
            if n == count:
                stack.append(cur)
                break
            count += 1
            pre, cur, nex = cur, cur.next, nex.next
        if n == count:
            # 判断是不是反转到最后一一个节点
            stack.append(cur)
        cur = stack.pop()
        if pre:
            # 判断是否是从第一个节点开始反转
            pre.next = cur
        else:
            head = cur
        while stack:
            cur.next = stack.pop()
            cur = cur.next
        cur.next = nex

        return head