LC.232. Implement Queue using Stacks(use two stacks)

https://leetcode.com/problems/implement-queue-using-stacks/description/
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).

worse case出现在 所有元素都在stack1 中，第一次 从 stack2 中 pop 

 1 public class LC_232_ImplementQueueUsingStacks {
 2     private Deque<Integer> stack1;
 3     private Deque<Integer> stack2;
 4 
 5     /**
 6      * Initialize your data structure here.
 7      */
 8     public LC_232_ImplementQueueUsingStacks() {
 9         stack1 = new LinkedList<>();
10         stack2 = new LinkedList<>();
11     }
12 
13     /**
14      * Push element x to the back of queue.
15      */
16     public void push(int x) {
17         stack1.push(x);
18     }
19 
20     /**
21      * Removes the element from in front of queue and returns that element.
22      */
23     public int pop() {
24         if (!stack2.isEmpty()) {
25             return stack2.pop();
26         } else {
27             shuffle();
28             return stack2.pop();
29         }
30     }
31 
32     /**
33      * Get the front element.
34      */
35     public int peek() {
36         if (!stack2.isEmpty()) {
37             return stack2.peek();
38         } else {
39             shuffle();
40             return stack2.peek();
41         }
42     }
43 
44     private void shuffle() {
45         while (!stack1.isEmpty()) {
46             stack2.push(stack1.pop());
47         }
48     }
49 
50     /**
51      * Returns whether the queue is empty.
52      */
53     public boolean empty() {
54         return stack1.isEmpty() && stack2.isEmpty();
55     }
56 }

 

转载于:https://www.cnblogs.com/davidnyc/p/8598787.html