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.
分析：
in栈只进，out栈只出。out空了从In导入
class MyQueue {
Stack<Integer> in, out;
/** Initialize your data structure here. */
public MyQueue() {
in = new Stack<Integer>();
out = new Stack<Integer>();
}
/** Push element x to the back of queue. */
public void push(int x) {
in.push(x);
}
/** Removes the element from in front of queue and returns that element. */
public int pop() {
if(out.empty()){
while(!in.empty()){
out.push(in.pop());
}
}
return out.pop();
}
/** Get the front element. */
public int peek() {
if(out.empty()){
while(!in.empty()){
out.push(in.pop());
}
}
return out.peek();
}
/** Returns whether the queue is empty. */
public boolean empty() {
return in.empty() && out.empty();
}
}
/**
* Your MyQueue object will be instantiated and called as such:
* MyQueue obj = new MyQueue();
* obj.push(x);
* int param_2 = obj.pop();
* int param_3 = obj.peek();
* boolean param_4 = obj.empty();
*/