Design Browser History

1472.Design-Browser-History/memo.md

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+# 1472. Design Browser History
+
+## step1
+まずlistを使ってとく。
+
+step1_del.py: 最初にlistの要素を消去することを考えたが、配列のコピーが発生する
+
+step1_del_v2.py: コピーを防ぐために del を使用
+
+step1_overwrite.py: lengthをクラスで管理する
+
+実行時間的にはstep1_overwrite.pyがもっとも速いが、不要なメモリを解放できることを考えるとstep1_del_v2.pyがもっとも良いと思った。
+
+LinkedListを使ってもとく。この場合はGCによってメモリが自然に解放される。ただし、履歴の移動でO(steps)かかる。
+
+## step2
+
+https://leetcode.com/problems/design-browser-history/solutions/674486/two-stacks-pretty-code-by-interviewrecip-kbz8/
+
+two stackを使った解法。履歴の移動でO(steps)かかる。
+
+以下の問題でも見たことがある。
+
+https://leetcode.com/problems/implement-queue-using-stacks/description/
+
+自分で思いつくことができなかった。
+
+過去と未来を行き来するような状況で発想できるようにしたい。
+
+
+https://chromium.googlesource.com/chromium/src/+/main/docs/session_history.md
+
+実際のブラウザがどうなっているか少し調べた。
+
+> Chromium tracks the session history of each tab in NavigationController, using a list of NavigationEntry objects to represent the joint session history items. Each frame creates session history items as it navigates. A joint session history item contains the state of each frame of a page at a given point in time, including things like URL, partially entered form data, scroll position, etc.
+
+リストとして管理しているようなので、配列の解法が最も近そう
+
+> Each NavigationEntry uses a tree of FrameNavigationEntries to track this state.
+
+Webページでは内部に複数のiframeがネストされているので、木構造で管理している
+

1472.Design-Browser-History/step1_del.py

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+class BrowserHistory:
+
+    def __init__(self, homepage: str):
+        self.history = [homepage]
+        self.index = 0
+
+    def visit(self, url: str) -> None:
+        self.history = self.history[:self.index + 1]
+        self.history.append(url)
+        self.index += 1
+
+    def back(self, steps: int) -> str:
+        self.index = max(0, self.index - steps)
+        return self.history[self.index]
+
+    def forward(self, steps: int) -> str:
+        self.index = min(len(self.history) - 1, self.index + steps)
+        return self.history[self.index]
+
+
+
+# Your BrowserHistory object will be instantiated and called as such:
+# obj = BrowserHistory(homepage)
+# obj.visit(url)
+# param_2 = obj.back(steps)
+# param_3 = obj.forward(steps)

1472.Design-Browser-History/step1_del_v2.py

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+class BrowserHistory:
+
+    def __init__(self, homepage: str):
+        self.history = [homepage]
+        self.index = 0
+
+    def visit(self, url: str) -> None:
+        del self.history[self.index + 1:]
+        self.history.append(url)
+        self.index += 1
+
+    def back(self, steps: int) -> str:
+        self.index = max(0, self.index - steps)
+        return self.history[self.index]
+
+    def forward(self, steps: int) -> str:
+        self.index = min(len(self.history) - 1, self.index + steps)
+        return self.history[self.index]
+
+
+
+# Your BrowserHistory object will be instantiated and called as such:
+# obj = BrowserHistory(homepage)
+# obj.visit(url)
+# param_2 = obj.back(steps)
+# param_3 = obj.forward(steps)

1472.Design-Browser-History/step1_linked_list.py

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+class Node:
+
+    def __init__(self, val: str):
+        self.val = val
+        self.prev = None
+        self.next = None
+
+
+class BrowserHistory:
+
+    def __init__(self, homepage: str):
+        self.node = Node(homepage)
+
+    def visit(self, url: str) -> None:
+        next_node = Node(url)
+        self.node.next = next_node
+        next_node.prev = self.node
+        self.node = next_node
+
+    def back(self, steps: int) -> str:
+        while steps > 0 and self.node.prev:
+            self.node = self.node.prev
+            steps -= 1
+        return self.node.val
+
+    def forward(self, steps: int) -> str:
+        while steps > 0 and self.node.next:
+            self.node = self.node.next
+            steps -= 1
+        return self.node.val
+
+
+# Your BrowserHistory object will be instantiated and called as such:
+# obj = BrowserHistory(homepage)
+# obj.visit(url)
+# param_2 = obj.back(steps)
+# param_3 = obj.forward(steps)

1472.Design-Browser-History/step1_overwrite.py

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+class BrowserHistory:
+
+    def __init__(self, homepage: str):
+        self.history = [homepage]
+        self.index = 0
+        self.length = 1
+
+    def visit(self, url: str) -> None:
+        if len(self.history) == self.index + 1:
+            self.history.append(url)
+        else:
+            self.history[self.index + 1] = url
+        self.index += 1
+        self.length = self.index + 1
+
+    def back(self, steps: int) -> str:
+        self.index = max(0, self.index - steps)
+        return self.history[self.index]
+
+    def forward(self, steps: int) -> str:
+        self.index = min(self.length - 1, self.index + steps)
+        return self.history[self.index]
+
+
+
+# Your BrowserHistory object will be instantiated and called as such:
+# obj = BrowserHistory(homepage)
+# obj.visit(url)
+# param_2 = obj.back(steps)
+# param_3 = obj.forward(steps)

1472.Design-Browser-History/step2_two_stack.py

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+class BrowserHistory:
+
+    def __init__(self, homepage: str):
+        self.back_stack = [homepage]
+        self.forward_stack = []
+
+    def visit(self, url: str) -> None:
+        self.back_stack.append(url)
+        self.forward_stack = []
+
+    def back(self, steps: int) -> str:
+        while steps > 0 and len(self.back_stack) > 1:
+            self.forward_stack.append(self.back_stack.pop())
+            steps -= 1
+        return self.back_stack[-1]
+
+    def forward(self, steps: int) -> str:
+        while steps > 0 and self.forward_stack:
+            self.back_stack.append(self.forward_stack.pop())
+            steps -= 1
+        return self.back_stack[-1]
+
+
+
+# Your BrowserHistory object will be instantiated and called as such:
+# obj = BrowserHistory(homepage)
+# obj.visit(url)
+# param_2 = obj.back(steps)
+# param_3 = obj.forward(steps)