126. Implement Queue using Stacks
阿新 • • 發佈：2018-08-30
返回 col lan win nat 雙端 是否為空 peek 切換
題目：
Implement the following operations of a queue using stacks.
使用堆棧實現隊列的以下操作。
- push(x) -- Push element x to the back of queue. 將元素x推送到隊列的後面。
- pop() -- Removes the element from in front of queue.從隊列前面刪除元素。
- peek() -- Get the front element.獲取前面的元素。
- empty() -- Return whether the queue is empty. 返回隊列是否為空。
Example:
MyQueue queue = new MyQueue(); queue.push(1); queue.push(2); queue.peek(); // returns 1 queue.pop(); // returns 1 queue.empty(); // returns false
Notes:
- You must use only standard operations of a stack -- which means only
push to top,peek/pop from top,size, andis emptyoperations are valid.您必須僅使用一個堆棧的標準操作 - 這意味著只能從頂部切換到頂部，查看/彈出，大小，並且空操作是有效的。 - Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.根據您的語言，本機可能不支持堆棧。 您可以使用列表或雙端隊列（雙端隊列）來模擬堆棧，只要您只使用堆棧的標準操作即可。
- You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).您可以假設所有操作都是有效的（例如，不會在空隊列上調用pop或Peek操作）。
解答：
1 class MyQueue { 2 3 private Stack<Integer> input; 4 private Stack<Integer> output; 5 6 /** Initialize your data structure here. */ 7 public MyQueue() { 8 input = new Stack<>(); 9 output = new Stack<>(); 10 } 11 12 /** Push element x to the back of queue. */ 13 public void push(int x) { 14 input.push(x); 15 } 16 17 /** Removes the element from in front of queue and returns that element. */ 18 public int pop() { 19 move(); 20 return output.pop(); 21 } 22 23 /** Get the front element. */ 24 public int peek() { 25 move(); 26 return output.peek(); 27 } 28 29 /** Returns whether the queue is empty. */ 30 public boolean empty() { 31 return input.isEmpty()&&output.isEmpty(); 32 } 33 34 private void move() { 35 if (output.isEmpty()) 36 while (!input.isEmpty()) 37 output.push(input.pop()); 38 } 39 } 40 41 /** 42 * Your MyQueue object will be instantiated and called as such: 43 * MyQueue obj = new MyQueue(); 44 * obj.push(x); 45 * int param_2 = obj.pop(); 46 * int param_3 = obj.peek(); 47 * boolean param_4 = obj.empty(); 48 */
詳解：
1.隊列：先進先出。 棧：先進後出
2.對於四個方法，隊列的push和empty方法與棧相同，只是棧的pop和peek是針對隊尾元素，而隊列的pop和peek是針對隊首元素
126. Implement Queue using Stacks