1472. Design Browser History
設計出瀏覽器的歷史記錄功能，包含：
1.BrowserHistory(string homepage)
初始化瀏覽器類別。
2.void visit(string url)
訪問url位置，並清空所有下一頁。
3.string back(int steps)
回到歷史記錄的上一頁並返回當前頁面，如果頁數不夠則返回最底頁。
4.string forward(int steps)
回到歷史記錄的下一頁並返回當前頁面，如果頁數不夠則返回最底頁。
Example:
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"
思路：
1.用一個雙向鏈結串列表示歷史記錄，back和forward就只是移動指標，visit就new一個
新的node並把右邊的連結打斷即可。
Java Code：