标签:visit back 1472 Design forward steps History com history
原题链接在这里:https://leetcode.com/problems/design-browser-history/
题目:
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 thehomepage
of the browser.void visit(string url)
Visitsurl
from the current page. It clears up all the forward history.string back(int steps)
Movesteps
back in history. If you can only returnx
steps in the history andsteps > x
, you will return onlyx
steps. Return the currenturl
after moving back in history at moststeps
.string forward(int steps)
Movesteps
forward in history. If you can only forwardx
steps in the history andsteps > x
, you will forward onlyx
steps. Return the currenturl
after forwarding in history at moststeps
.
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"
Constraints:
1 <= homepage.length <= 20
1 <= url.length <= 20
1 <= steps <= 100
homepage
andurl
consist of '.' or lower case English letters.- At most
5000
calls will be made tovisit
,back
, andforward
.
题解:
We could use two stacks to maintain the history and future.
There must always be at least one element in the history stack.
Time Complexity: visit, O(1). back, O(steps). forward, O(steps).
Space: O(n). n = history + future size.
AC Java:
1 class BrowserHistory { 2 Stack<String> history; 3 Stack<String> future; 4 5 public BrowserHistory(String homepage) { 6 history = new Stack<>(); 7 future = new Stack<>(); 8 history.push(homepage); 9 } 10 11 public void visit(String url) { 12 history.push(url); 13 future = new Stack<>(); 14 } 15 16 public String back(int steps) { 17 while(history.size() > 1 && steps > 0){ 18 future.push(history.pop()); 19 steps--; 20 } 21 22 return history.peek(); 23 } 24 25 public String forward(int steps) { 26 while(future.size() > 0 && steps > 0){ 27 history.push(future.pop()); 28 steps--; 29 } 30 31 return history.peek(); 32 } 33 } 34 35 /** 36 * Your BrowserHistory object will be instantiated and called as such: 37 * BrowserHistory obj = new BrowserHistory(homepage); 38 * obj.visit(url); 39 * String param_2 = obj.back(steps); 40 * String param_3 = obj.forward(steps); 41 */
Could use a linkedList as well.
Time Complexity: visit, O(1). back, O(n). forward, O(n). n = history.size().
Space: O(n).
AC Java:
1 class BrowserHistory { 2 LinkedList<String> history; 3 int index; 4 5 public BrowserHistory(String homepage) { 6 history = new LinkedList<>(); 7 history.addLast(homepage); 8 index = 0; 9 } 10 11 public void visit(String url) { 12 history.subList(index + 1, history.size()).clear(); 13 history.addLast(url); 14 index++; 15 } 16 17 public String back(int steps) { 18 index = index - steps < 0 ? 0 : index - steps; 19 return history.get(index); 20 } 21 22 public String forward(int steps) { 23 index = index + steps >= history.size() ? history.size() - 1 : index + steps; 24 return history.get(index); 25 } 26 } 27 28 /** 29 * Your BrowserHistory object will be instantiated and called as such: 30 * BrowserHistory obj = new BrowserHistory(homepage); 31 * obj.visit(url); 32 * String param_2 = obj.back(steps); 33 * String param_3 = obj.forward(steps); 34 */
标签:visit,back,1472,Design,forward,steps,History,com,history 来源: https://www.cnblogs.com/Dylan-Java-NYC/p/16611589.html
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