LeetCode Implement Queue using Stacks

原题链接在这里：https://leetcode.com/problems/implement-queue-using-stacks/

题目：

Implement the following operations of a queue using stacks.

• push(x) -- Push element x to the back of queue.
• pop() -- Removes the element from in front of queue.
• peek() -- Get the front element.
• empty() -- Return whether the queue is empty.

Notes:

• You must use only standard operations of a stack -- which means only push to top, peek/pop from top, size, and is empty operations are valid.
• 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.
• You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

题解：

与Implement Stack using Queues相对应.

用Stack implement queue时，可以采用两个stack, add时就是一直像stk1中压栈。

poll()时把stk1的所有元素逐个压入另一个stack stk2中，现在stk2最上面的元素就是最先来的的, pop()之后再把剩下的压回到stk1中。

peek()与poll()类似，但这次不用pop(), 只需peek()出一个值返回。

Time Complexity: push, O(1). pop, O(n), n is current number of integers in stack. peek O(n). empty O(1).

Space: O(n), 两个stack.

AC Java:

 1 public class MyQueue {
 2     Stack<Integer> stk1;
 3     Stack<Integer> stk2;
 4     
 5     /** Initialize your data structure here. */
 6     public MyQueue() {
 7         stk1 = new Stack<Integer>();
 8         stk2 = new Stack<Integer>();
 9     }
10     
11     /** Push element x to the back of queue. */
12     public void push(int x) {
13         stk1.push(x);
14     }
15     
16     /** Removes the element from in front of queue and returns that element. */
17     public int pop() {
18         while(!stk1.isEmpty()){
19             stk2.push(stk1.pop());
20         }
21         int res = 0;
22         if(!stk2.isEmpty()){
23             res = stk2.pop();
24         }
25         while(!stk2.isEmpty()){
26             stk1.push(stk2.pop());
27         }
28         return res;
29     }
30     
31     /** Get the front element. */
32     public int peek() {
33         while(!stk1.isEmpty()){
34             stk2.push(stk1.pop());
35         }
36         int res = 0;
37         if(!stk2.isEmpty()){
38             res = stk2.peek();
39         }
40         while(!stk2.isEmpty()){
41             stk1.push(stk2.pop());
42         }
43         return res;
44     }
45     
46     /** Returns whether the queue is empty. */
47     public boolean empty() {
48         return stk1.isEmpty();
49     }
50 }
51 
52 /**
53  * Your MyQueue object will be instantiated and called as such:
54  * MyQueue obj = new MyQueue();
55  * obj.push(x);
56  * int param_2 = obj.pop();
57  * int param_3 = obj.peek();
58  * boolean param_4 = obj.empty();
59  */

 

转载于:https://www.cnblogs.com/Dylan-Java-NYC/p/4825030.html