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@@ -1 +1,261 @@
 # Step1
+
+## アプローチ
+
+* 末尾の要素にすぐアクセスしたいし, 先頭にすぐ持っていきたい
+* LinkedListとして保存
+* 双方向にlinkを作っておく
+* キーに対応するLinkedListNodeを作っておく
+* 2:37
+
+## Code1-1
+
+* AC: 16:30
+
+```python
+
+class Node:
+    def __init__(self, value = None, key = None, next_node = None, previous_node = None):
+        self.value = value
+        self.key = key
+        self.next_node = next_node
+        self.previous_node = previous_node
+
+class LRUCache:
+
+    def __init__(self, capacity: int):
+        self.capacity = capacity
+
+        self.key_to_node = {}
+
+        self.head = Node()
+        self.tail =  Node()
+        self.head.next_node = self.tail
+        self.tail.previous_node = self.head
+
+
+    def _delete_relation(self, node) -> None:
+        previous_node = node.previous_node
+        next_node = node.next_node
+
+        previous_node.next_node = next_node
+        next_node.previous_node = previous_node
+        return
+
+    def _add_head_relation(self, node) -> None:
+        original_first_node = self.head.next_node
+
+        self.head.next_node = node
+        node.previous_node = self.head
+
+        node.next_node = original_first_node
+        original_first_node.previous_node = node
+
+        return
+
+    def get(self, key: int) -> int:
+        if key not in self.key_to_node:
+            return -1
+        node = self.key_to_node[key]
+        self._delete_relation(node)
+        self._add_head_relation(node)
+        return node.value
+        
+
+    def put(self, key: int, value: int) -> None:
+        if key in self.key_to_node:
+            node = self.key_to_node[key]
+            node.value = value
+            self.get(key)
+            return
+
+        if len(self.key_to_node) >= self.capacity:
+            tail_node = self.tail.previous_node
+            self._delete_relation(tail_node)
+            del self.key_to_node[tail_node.key]
+
+        node = Node(value, key)
+        self.key_to_node[key] = node
+        self._add_head_relation(node)
+        return
+
+```
+
+# Step2
+
+## Code2-1
+
+* listの管理を別のクラスで行うことにした
+
+```python
+class Node:
+    def __init__(self, value = None, key = None, next_node = None, previous_node = None):
+        self.value = value
+        self.key = key
+        self.next_node = next_node
+        self.previous_node = previous_node
+
+
+class DoublyLinkedList:
+    def __init__(self):
+        self.head = Node()
+        self.tail =  Node()
+        self.head.next_node = self.tail
+        self.tail.previous_node = self.head
+
+    def insert_between(self, node, previous_node, next_node):
+        previous_node.next_node = node
+        node.next_node = next_node
+        next_node.previous_node = node
+        node.previous_node = previous_node
+        return
+
+    def delete(self, node):
+        previous_node = node.previous_node
+        next_node = node.next_node
+        previous_node.next_node = next_node
+        next_node.previous_node = previous_node
+        return
+
+    def pop_tail(self):
+        node_to_delete = self.tail.previous_node
+        self.delete(node_to_delete)
+        return node_to_delete
+
+    def append_front(self, node):
+        self.insert_between(node, self.head, self.head.next_node)
+
+    def move_to_front(self, node):
+        self.delete(node)
+        self.append_front(node)
+
+
+class LRUCache:
+
+    def __init__(self, capacity: int):
+        self.capacity = capacity
+        self.key_to_node = {}
+        self.nodes = DoublyLinkedList()
+
+    def get(self, key: int) -> int:
+        if key not in self.key_to_node:
+            return -1
+        node = self.key_to_node[key]
+        self.nodes.move_to_front(node)
+        return node.value
+
+    def put(self, key: int, value: int) -> None:
+        if key in self.key_to_node:
+            node = self.key_to_node[key]
+            node.value = value
+            self.nodes.move_to_front(node)
+            return
+
+        if len(self.key_to_node) >= self.capacity:
+            node = self.nodes.pop_tail()
+            del self.key_to_node[node.key]
+
+        node = Node(value, key)
+        self.key_to_node[key] = node
+        self.nodes.append_front(node)
+        return
+
+```
+
+## 他の人のPRを見る
+
+* https://github.com/tom4649/Coding/pull/72
+    * OrderedDictを使うとシンプルに描ける
+    * > 将来拡張するときにバグが出にくくなりそうだなと思うので、一応自分ならnode.nextとnode.prevにNoneを入れておくかなと思いました。
+* https://github.com/t0hsumi/leetcode/pull/16
+    * 自分と同じように別でデータ構造(このPRではDeque)をクラスで定義している
+* https://github.com/Mike0121/LeetCode/pull/49
+    * > 仮に、これがインタビューで聞かれて OrderedDict で書けますが返事だとしたら、中を教えて下さいになると思います。
+
+# Step3
+
+## Code3-1
+
+```python
+class Node:
+    def __init__(self, key = -1, value = -1, next = None, previous = None):
+        self.key = key
+        self.value = value
+        self.next = next
+        self.previous = previous
+
+class DoubleLinkedList:
+    def __init__(self):
+        self.head = Node()
+        self.tail = Node()
+        self.head.next = self.tail
+        self.tail.previous = self.head
+
+
+    def insert_in_front(self, node):
+        front_node = self.head.next
+
+        node.previous = self.head
+        node.next = front_node
+
+        self.head.next = node
+
+        front_node.previous = node
+
+        return
+    
+    def delete(self, node):
+        if node.next is None or node.previous is None:
+            return
+        
+        previous_node = node.previous
+        next_node = node.next
+
+        previous_node.next = next_node
+        next_node.previous = previous_node
+
+        node.next = None
+        node.previous = None
+
+        return node
+
+    def delete_last(self):
+        return self.delete(self.tail.previous)
+
+
+class LRUCache:
+
+    def __init__(self, capacity: int):
+        self.capacity = capacity
+        self.key_to_node = {}
+        self.nodes = DoubleLinkedList()
+
+    def get(self, key: int) -> int:
+        if key not in self.key_to_node:
+            return -1
+        
+        node = self.key_to_node[key]
+        self.nodes.delete(node)
+        self.nodes.insert_in_front(node)
+
+        return node.value
+        
+
+    def put(self, key: int, value: int) -> None:
+        if key in self.key_to_node:
+            node = self.key_to_node[key]
+            node.value = value
+            self.nodes.delete(node)
+            self.nodes.insert_in_front(node)
+            return
+        
+        if len(self.key_to_node) >= self.capacity:
+            last_node = self.nodes.delete_last()
+            del self.key_to_node[last_node.key]
+        
+        node = Node(key, value)
+        self.key_to_node[key] = node
+        self.nodes.insert_in_front(node)
+        return
+
+```
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