添加 0707.设计链表 Python3版本

problems/0707.设计链表.md

@@ -231,8 +231,166 @@ class MyLinkedList {
 ```
 
 Python：
+```python3
+# 单链表
+class Node:
+    
+    def __init__(self, val):
+        self.val = val
+        self.next = None
 
 
+class MyLinkedList:
+
+    def __init__(self):
+        self._head = Node(0)  # 虚拟头部节点
+        self._count = 0  # 添加的节点数
+
+    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.
+        """
+        if 0 <= index < self._count:
+            node = self._head
+            for _ in range(index + 1):
+                node = node.next
+            return node.val
+        else:
+            return -1
+
+    def addAtHead(self, val: int) -> None:
+        """
+        Add a node of value val before the first element of the linked list. After the insertion, the new node will be the first node of the linked list.
+        """
+        self.addAtIndex(0, val)
+
+    def addAtTail(self, val: int) -> None:
+        """
+        Append a node of value val to the last element of the linked list.
+        """
+        self.addAtIndex(self._count, val)
+
+    def addAtIndex(self, index: int, val: int) -> None:
+        """
+        Add a node of value val before the index-th node in the linked list. If index equals to the length of linked list, the node will be appended to the end of linked list. If index is greater than the length, the node will not be inserted.
+        """
+        if index < 0:
+            index = 0
+        elif index > self._count:
+            return
+
+        # 计数累加
+        self._count += 1
+
+        add_node = Node(val)
+        prev_node, current_node = None, self._head
+        for _ in range(index + 1):
+            prev_node, current_node = current_node, current_node.next
+        else:
+            prev_node.next, add_node.next = add_node, current_node
+
+    def deleteAtIndex(self, index: int) -> None:
+        """
+        Delete the index-th node in the linked list, if the index is valid.
+        """
+        if 0 <= index < self._count:
+            # 计数-1
+            self._count -= 1
+            prev_node, current_node = None, self._head
+            for _ in range(index + 1):
+                prev_node, current_node = current_node, current_node.next
+            else:
+                prev_node.next, current_node.next = current_node.next, None
+
+
+# 双链表
+# 相对于单链表, Node新增了prev属性
+class Node:
+    
+    def __init__(self, val):
+        self.val = val
+        self.prev = None
+        self.next = None
+
+
+class MyLinkedList:
+
+    def __init__(self):
+        self._head, self._tail = Node(0), Node(0)  # 虚拟节点
+        self._head.next, self._tail.prev = self._tail, self._head
+        self._count = 0  # 添加的节点数
+
+    def _get_node(self, index: int) -> Node:
+        # 当index小于_count//2时, 使用_head查找更快, 反之_tail更快
+        if index >= self._count // 2:
+            # 使用prev往前找
+            node = self._tail
+            for _ in range(self._count - index):
+                node = node.prev
+        else:
+            # 使用next往后找
+            node = self._head   
+            for _ in range(index + 1):
+                node = node.next
+        return node
+
+    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.
+        """
+        if 0 <= index < self._count:
+            node = self._get_node(index)
+            return node.val
+        else:
+            return -1
+
+    def addAtHead(self, val: int) -> None:
+        """
+        Add a node of value val before the first element of the linked list. After the insertion, the new node will be the first node of the linked list.
+        """
+        self._update(self._head, self._head.next, val)
+
+    def addAtTail(self, val: int) -> None:
+        """
+        Append a node of value val to the last element of the linked list.
+        """
+        self._update(self._tail.prev, self._tail, val)
+
+    def addAtIndex(self, index: int, val: int) -> None:
+        """
+        Add a node of value val before the index-th node in the linked list. If index equals to the length of linked list, the node will be appended to the end of linked list. If index is greater than the length, the node will not be inserted.
+        """
+        if index < 0:
+            index = 0
+        elif index > self._count:
+            return
+        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.
+        """
+        if 0 <= index < self._count:
+            node = self._get_node(index)
+            # 计数-1
+            self._count -= 1
+            node.prev.next, node.next.prev = node.next, node.prev
+```
+
 Go：