Implement Stack using Queues 解答

 1 class MyStack {
 2     Queue<Integer> queue1;
 3     Queue<Integer> queue2;
 4     
 5     public MyStack(){
 6         queue1 = new LinkedList<Integer>();
 7         queue2 = new LinkedList<Integer>();
 8     }
 9     
10     // Push element x onto stack.
11     public void push(int x) {
12         queue1.add(x);
13     }
14 
15     // Removes the element on top of the stack.
16     public void pop() {
17         if (queue1.peek() == null)
18             return;
19         int length = queue1.size();
20         queue2 = new LinkedList<Integer>();
21         while (length > 1) {
22             queue2.add(queue1.remove());
23             length--;
24         }
25         queue1 = queue2;
26     }
27 
28     // Get the top element.
29     public int top() {
30         if (queue1.peek() == null)
31             return -1;
32         queue2 = new LinkedList<Integer>();
33         int length = queue1.size();
34         int lastElement = 0;
35         while (length > 0) {
36             lastElement = queue1.remove();
37             queue2.add(lastElement);
38             length--;
39         }
40         queue1 = queue2;
41         return lastElement;
42     }
43 
44     // Return whether the stack is empty.
45     public boolean empty() {
46         if (queue1.peek() == null)
47             return true;
48         return false;
49     }
50 }