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###### tags: `Leetcode`
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# Leetcode 1472. Design Browser History

[link](https://leetcode.com/problems/design-browser-history/)

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You have a browser of one tab where you start on the homepage and you can visit another url, get back in the history number of steps or move forward in the history number of steps.

Implement the BrowserHistory class:

- BrowserHistory(string homepage) Initializes the object with the homepage of the browser.
- void visit(string url) Visits url from the current page. It clears up all the forward history.
- string back(int steps) Move steps back in history. If you can only return x steps in the history and steps > x, you will return only x steps. Return the current url after moving back in history at most steps.
- string forward(int steps) Move steps forward in history. If you can only forward x steps in the history and steps > x, you will forward only x steps. Return the current url after forwarding in history at most steps.

#### Example 1:

Input:

["BrowserHistory","visit","visit","visit","back","back","forward","visit","forward","back","back"]
[["leetcode.com"],["google.com"],["facebook.com"],["youtube.com"],[1],[1],[1],["linkedin.com"],[2],[2],[7]]

Output:

[null,null,null,null,"facebook.com","google.com","facebook.com",null,"linkedin.com","google.com","leetcode.com"]

Explanation:

BrowserHistory browserHistory = new BrowserHistory("leetcode.com");
browserHistory.visit("google.com");       // You are in "leetcode.com". Visit "google.com"
browserHistory.visit("facebook.com");     // You are in "google.com". Visit "facebook.com"
browserHistory.visit("youtube.com");      // You are in "facebook.com". Visit "youtube.com"
browserHistory.back(1);                   // You are in "youtube.com", move back to "facebook.com" return "facebook.com"
browserHistory.back(1);                   // You are in "facebook.com", move back to "google.com" return "google.com"
browserHistory.forward(1);                // You are in "google.com", move forward to "facebook.com" return "facebook.com"
browserHistory.visit("linkedin.com");     // You are in "facebook.com". Visit "linkedin.com"
browserHistory.forward(2);                // You are in "linkedin.com", you cannot move forward any steps.
browserHistory.back(2);                   // You are in "linkedin.com", move back two steps to "facebook.com" then to "google.com". return "google.com"
browserHistory.back(7);                   // You are in "google.com", you can move back only one step to "leetcode.com". return "leetcode.com"

#### Constraints:

- 1 <= homepage.length <= 20
- 1 <= url.length <= 20
- 1 <= steps <= 100
- homepage and url consist of '.' or lower case English letters.
- At most 5000 calls will be made to visit, back, and forward.

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This code defines two classes: ListNode and BrowserHistory.

The ListNode class represents a node in a doubly linked list. Each node contains a value (val), a reference to the previous node (prev), and a reference to the next node (next).

The BrowserHistory class represents a simplified browser history. It has the following methods:

__init__(self, homepage: str): Initializes the browser history with a cur node pointing to the homepage passed as an argument.

visit(self, url: str) -> None: Adds a new node with the given URL after the current node. The new node becomes the current node.

back(self, steps: int) -> str: Moves the current node backward by the specified number of steps. If there are no nodes to move back to, the current node remains unchanged. Returns the URL of the new current node.

forward(self, steps: int) -> str: Moves the current node forward by the specified number of steps. If there are no nodes to move forward to, the current node remains unchanged. Returns the URL of the new current node.

In this implementation, the browser history is represented using a doubly linked list, where each node represents a visited URL. The visit method adds a new node when a new URL is visited, and the back and forward methods traverse the list by updating the cur node.

#### Solution 1

class ListNode:
    def __init__(self, val, prev=None, next=None):
        self.val = val
        self.prev = prev
        self.next = next


class BrowserHistory:

    def __init__(self, homepage: str):
        self.cur = ListNode(homepage)


    def visit(self, url: str) -> None:
        self.cur.next = ListNode(url, self.cur)
        self.cur = self.cur.next

    def back(self, steps: int) -> str:
        while self.cur.prev and steps > 0:
            self.cur = self.cur.prev
            steps -= 1

        return self.cur.val


    def forward(self, steps: int) -> str:
        while self.cur.next and steps > 0:
            self.cur = self.cur.next
            steps -= 1

        return self.cur.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)