你有一个只支持单个标签页的 浏览器 ，最开始你浏览的网页是 homepage ，你可以访问其他的网站 url ，也可以在浏览历史中后退 steps 步或前进 steps 步。
请你实现 BrowserHistory 类：
BrowserHistory(string homepage)，用homepage初始化浏览器类。void visit(string url)从当前页跳转访问url对应的页面 。执行此操作会把浏览历史前进的记录全部删除。string back(int steps)在浏览历史中后退steps步。如果你只能在浏览历史中后退至多x步且steps > x，那么你只后退x步。请返回后退 至多steps步以后的url。string forward(int steps)在浏览历史中前进steps步。如果你只能在浏览历史中前进至多x步且steps > x，那么你只前进x步。请返回前进 至多steps步以后的url。
示例：
输入：
["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]]
输出：
[null,null,null,null,"facebook.com","google.com","facebook.com",null,"linkedin.com","google.com","leetcode.com"]
解释：
BrowserHistory browserHistory = new BrowserHistory("leetcode.com");
browserHistory.visit("google.com"); // 你原本在浏览 "leetcode.com" 。访问 "google.com"
browserHistory.visit("facebook.com"); // 你原本在浏览 "google.com" 。访问 "facebook.com"
browserHistory.visit("youtube.com"); // 你原本在浏览 "facebook.com" 。访问 "youtube.com"
browserHistory.back(1); // 你原本在浏览 "youtube.com" ，后退到 "facebook.com" 并返回 "facebook.com"
browserHistory.back(1); // 你原本在浏览 "facebook.com" ，后退到 "google.com" 并返回 "google.com"
browserHistory.forward(1); // 你原本在浏览 "google.com" ，前进到 "facebook.com" 并返回 "facebook.com"
browserHistory.visit("linkedin.com"); // 你原本在浏览 "facebook.com" 。 访问 "linkedin.com"
browserHistory.forward(2); // 你原本在浏览 "linkedin.com" ，你无法前进任何步数。
browserHistory.back(2); // 你原本在浏览 "linkedin.com" ，后退两步依次先到 "facebook.com" ，然后到 "google.com" ，并返回 "google.com"
browserHistory.back(7); // 你原本在浏览 "google.com"， 你只能后退一步到 "leetcode.com" ，并返回 "leetcode.com"
提示：
1 <= homepage.length <= 201 <= url.length <= 201 <= steps <= 100homepage和url都只包含 '.' 或者小写英文字母。- 最多调用
5000次visit，back和forward函数。
列表实现。
class BrowserHistory:
def __init__(self, homepage: str):
self.urls = []
self.cur = -1
self.tail = -1
self.visit(homepage)
def visit(self, url: str) -> None:
self.cur += 1
if self.cur < len(self.urls):
self.urls[self.cur] = url
else:
self.urls.append(url)
self.tail = self.cur
def back(self, steps: int) -> str:
self.cur = max(0, self.cur -steps)
return self.urls[self.cur]
def forward(self, steps: int) -> str:
self.cur = min(self.tail, self.cur + steps)
return self.urls[self.cur]
# 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)栈实现。
class BrowserHistory:
def __init__(self, homepage: str):
self.s1 = []
self.s2 = []
self.cur = homepage
def visit(self, url: str) -> None:
self.s2.clear()
self.s1.append(self.cur)
self.cur = url
def back(self, steps: int) -> str:
while steps > 0 and self.s1:
self.s2.append(self.cur)
self.cur = self.s1.pop()
steps -= 1
return self.cur
def forward(self, steps: int) -> str:
while steps > 0 and self.s2:
self.s1.append(self.cur)
self.cur = self.s2.pop()
steps -= 1
return self.cur
# 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)列表实现。
class BrowserHistory {
private List<String> urls;
private int cur = -1;
private int tail = -1;
public BrowserHistory(String homepage) {
urls = new ArrayList<>();
visit(homepage);
}
public void visit(String url) {
++cur;
if (cur < urls.size()) {
urls.set(cur, url);
} else {
urls.add(url);
}
tail = cur;
}
public String back(int steps) {
cur = Math.max(0, cur - steps);
return urls.get(cur);
}
public String forward(int steps) {
cur = Math.min(tail, cur + steps);
return urls.get(cur);
}
}
/**
* Your BrowserHistory object will be instantiated and called as such:
* BrowserHistory obj = new BrowserHistory(homepage);
* obj.visit(url);
* String param_2 = obj.back(steps);
* String param_3 = obj.forward(steps);
*/栈实现。
class BrowserHistory {
private Deque<String> s1;
private Deque<String> s2;
private String cur;
public BrowserHistory(String homepage) {
s1 = new ArrayDeque<>();
s2 = new ArrayDeque<>();
cur = homepage;
}
public void visit(String url) {
s2.clear();
s1.push(cur);
cur = url;
}
public String back(int steps) {
while (steps > 0 && !s1.isEmpty()) {
s2.push(cur);
cur = s1.pop();
--steps;
}
return cur;
}
public String forward(int steps) {
while (steps > 0 && !s2.isEmpty()) {
s1.push(cur);
cur = s2.pop();
--steps;
}
return cur;
}
}
/**
* Your BrowserHistory object will be instantiated and called as such:
* BrowserHistory obj = new BrowserHistory(homepage);
* obj.visit(url);
* String param_2 = obj.back(steps);
* String param_3 = obj.forward(steps);
*/