regular leetcode

Author: KrisYu

092._reverse_linked_list_ii.md

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+###92. Reverse Linked List II 
+
+题目:
+<https://leetcode.com/problems/reverse-linked-list-ii/>
+
+
+难度:
+Medium
+
+
+跟 reverse linked list一样
+
+思路： 找到 第 m 个node，然后开始reverse到第n个node，然后再把它们和原本的list连接起来
+
+AC 代码
+
+```
+class Solution(object):
+    def reverseBetween(self, head, m, n):
+        """
+        :type head: ListNode
+        :type m: int
+        :type n: int
+        :rtype: ListNode
+        """
+        # m == n, not reverse
+        if m == n : return head
+
+        dummy = ListNode(-1)
+        dummy.next = head
+
+        mbefore = dummy
+        cnt = 1
+        
+        while mbefore and cnt < m:
+            mbefore = mbefore.next
+            cnt += 1
+
+        prev = None
+        cur = mbefore.next
+        tail1 = mbefore.next
+        
+
+        while cnt <= n :
+            nxt = cur.next
+            cur.next = prev
+            prev = cur
+            cur = nxt
+            cnt += 1
+
+
+
+        mbefore.next = prev
+        tail1.next = cur
+
+        return dummy.next 
+```
+
+看了一下别人的代码，又比我写的好嘛，因为是保证m和n有效，用的是for循环先找到 m node:
+
+
+	for _ in range(m-1):
+		....
+		
+	for _ in range(n-m):
+		reverse 操作