206. Reverse Linked List

Reverse Linked List/memo.md

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+# step1
+
+ループ処理の最中でノードを反転させていく方法が一番最初に思いついた。（というか、多分やったことがあって記憶にあった）
+2分くらいで手順を書き出して、それをコード化することができた。
+
+# step2
+
+ループでのアプローチ以外に
+- 再帰
+- スタック
+があることを知った。
+
+## 再帰について
+
+今回再帰でコード書いてみると通りはするけど、実際にノードの連結数に明記がないので不適。（LeetCodeの環境なら大丈夫なコードになってしまうから）
+
+時間計算量はO(N), 空間計算量はO(N)
+
+## スタック
+
+アプローチはとても自然で他人が書いたコードでも読みやすいと思った。
+ただ空間計算量がO(N)になり、他のアプローチを考えるとデメリット。
+
+時間計算量はO(N), 空間計算量はO(N)
+
+## step3
+
+AIもつかってリファクタリングをした。
+再帰はベースケースとエッジケースを最初に明記して後続の処理をよりシンプルに書くように変更
+
+

Reverse Linked List/step1.py

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+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+class Solution:
+    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        prev = None
+        node = head
+
+        while node:
+            tmp = node.next
+            node.next = prev
+            prev = node
+            node = tmp
+        return prev
+

Reverse Linked List/step2-using-iterative.py

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+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+class Solution:
+    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        previous_node, current_node = None, head
+
+        while current_node:
+            tmp = current_node.next
+            current_node.next = previous_node
+            previous_node = current_node
+            current_node = tmp
+        return previous_node
+

Reverse Linked List/step2-using-recursive.py

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+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+class Solution:
+    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        if not head:
+            return None
+
+        new_head = head
+
+        if head.next:
+            new_head = self.reverseList(head.next)
+            head.next.next = head
+        head.next = None
+
+        return new_head
+

Reverse Linked List/step2-using-stack.py

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+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+class Solution:
+    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        if not head:
+            return None
+
+        stack = []
+        node = head
+        while node:
+            stack.append(node)
+            node = node.next
+
+        new_head = stack.pop()
+        tail = new_head
+
+        while stack:
+            new_tail = stack.pop()
+            tail.next = new_tail
+            tail = tail.next
+
+        tail.next = None
+        return new_head
+

Reverse Linked List/step3-using-iteration.py

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+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+class Solution:
+    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        previous_node, current_node = None, head
+
+        while current_node:
+            tmp = current_node.next
+            current_node.next = previous_node
+            previous_node = current_node
+            current_node = tmp
+        return previous_node

Reverse Linked List/step3-using-recursion.py

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+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+class Solution:
+    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        if head is None or head.next is None:
+            return head
+
+        new_head = self.reverseList(head.next)
+        head.next.next = head
+        head.next = None
+
+        return new_head
+

Reverse Linked List/step3-using-stack.py

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+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+class Solution:
+    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        if head is None or head.next is None:
+            return head
+
+        stack = []
+        node = head
+
+        while node:
+            stack.append(node)
+            node = node.next
+
+        tail = stack.pop()
+        node = tail
+
+        while stack:
+            node.next = stack.pop()
+            node = node.next
+        node.next = None
+
+        return tail
+

Reverse Linked List/step4-using-iteration.py

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+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+class Solution:
+    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        if head is None or head.next is None:
+            return head
+
+        new_head_node, current_node = None, head
+
+        while current_node:
+            rest_node_head = current_node.next
+            current_node.next = new_head_node
+            new_head_node = current_node
+            current_node = rest_node_head
+        return new_head_node
+

Reverse Linked List/step4-using-recursion.py

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+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+class Solution:
+    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        if head is None or head.next is None:
+            return head
+
+        new_head = self.reverseList(head.next)
+        new_previous_node = head.next
+        new_previous_node.next = head
+        head.next = None
+        return new_head
+

Reverse Linked List/step4-using-stack.py

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+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+class Solution:
+    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        if head is None or head.next is None:
+            return head
+        stack = []
+        node = head
+        while node:
+            stack.append(node)
+            node = node.next
+        new_head = stack.pop()
+        node = new_head
+        while stack:
+            node.next = stack.pop()
+            node = node.next
+        node.next = None
+        return new_head