Update 0707.设计链表.md

修改 python 代码，dummy node

problems/0707.设计链表.md

@@ -486,15 +486,10 @@ class MyLinkedList {
 Python：
 ```python
 # 单链表
-class Node(object):
-    def __init__(self, x=0):
-        self.val = x
-        self.next = None
-
-class MyLinkedList(object):
+class MyLinkedList1:
 
     def __init__(self):
-        self.head = Node()
+        self.dummy_head = Node()# 添加虚拟头指针，便于操作
         self.size = 0 # 设置一个链表长度的属性，便于后续操作，注意每次增和删的时候都要更新
 
     def get(self, index):
@@ -504,7 +499,7 @@ class MyLinkedList(object):
         """
         if index < 0 or index >= self.size:
             return -1
-        cur = self.head.next
+        cur = self.dummy_head.next
         while(index):
             cur = cur.next
             index -= 1
@@ -516,8 +511,8 @@ class MyLinkedList(object):
         :rtype: None
         """
         new_node = Node(val)
-        new_node.next = self.head.next
-        self.head.next = new_node
+        new_node.next = self.dummy_head.next
+        self.dummy_head.next = new_node
         self.size += 1
 
     def addAtTail(self, val):
@@ -526,7 +521,7 @@ class MyLinkedList(object):
         :rtype: None
         """
         new_node = Node(val)
-        cur = self.head
+        cur = self.dummy_head
         while(cur.next):
             cur = cur.next
         cur.next = new_node
@@ -548,12 +543,12 @@ class MyLinkedList(object):
             return
 
         node = Node(val)
-        pre = self.head
+        cur = self.dummy_head
         while(index):
-            pre = pre.next
+            cur = cur.next
             index -= 1
-        node.next = pre.next
-        pre.next = node
+        node.next = cur.next
+        cur.next = node
         self.size += 1
 
     def deleteAtIndex(self, index):
@@ -563,7 +558,7 @@ class MyLinkedList(object):
         """
         if index < 0 or index >= self.size:
             return
-        pre = self.head
+        pre = self.dummy_head
         while(index):
             pre = pre.next
             index -= 1
@@ -574,11 +569,10 @@ class MyLinkedList(object):
 # 相对于单链表, Node新增了prev属性
 class Node:
 
-    def __init__(self, val):
+    def __init__(self, val=0, next = None, prev = None):
         self.val = val
-        self.prev = None
-        self.next = None
-
+        self.next = next
+        self.prev = prev
 
 class MyLinkedList:
 
@@ -601,6 +595,20 @@ class MyLinkedList:
                 node = node.next
         return node
 
+
+    def _update(self, prev: Node, next: Node, val: int) -> None:
+        """
+            更新节点
+            :param prev: 相对于更新的前一个节点
+            :param next: 相对于更新的后一个节点
+            :param val:  要添加的节点值
+        """
+        # 计数累加
+        self._count += 1
+        node = Node(val)
+        prev.next, next.prev = node, node
+        node.prev, node.next = prev, next
+
     def get(self, index: int) -> int:
         """
         Get the value of the index-th node in the linked list. If the index is invalid, return -1.
@@ -634,19 +642,6 @@ class MyLinkedList:
         node = self._get_node(index)
         self._update(node.prev, node, val)
 
-    def _update(self, prev: Node, next: Node, val: int) -> None:
-        """
-            更新节点
-            :param prev: 相对于更新的前一个节点
-            :param next: 相对于更新的后一个节点
-            :param val:  要添加的节点值
-        """
-        # 计数累加
-        self._count += 1
-        node = Node(val)
-        prev.next, next.prev = node, node
-        node.prev, node.next = prev, next
-
     def deleteAtIndex(self, index: int) -> None:
         """
         Delete the index-th node in the linked list, if the index is valid.
@@ -656,6 +651,16 @@ class MyLinkedList:
             # 计数-1
             self._count -= 1
             node.prev.next, node.next.prev = node.next, node.prev
+
+
+
+# Your MyLinkedList object will be instantiated and called as such:
+# obj = MyLinkedList()
+# param_1 = obj.get(index)
+# obj.addAtHead(val)
+# obj.addAtTail(val)
+# obj.addAtIndex(index,val)
+# obj.deleteAtIndex(index)
 ```
 
 Go：