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.
题意：用栈来实现队列。
思路：用一个栈来存放push进的元素，另一个栈存放要pop出的元素。当调用pop或peek时，如果pop栈是空的，就把push栈里的元素从栈顶一个个取出来push进pop栈，这样pop栈的栈顶就是最早push进来的元素。
class MyQueue {
private Stack<Integer> pushStack;
private Stack<Integer> popStack;
/** Initialize your data structure here. */
public MyQueue() {
this.pushStack = new Stack<>();
this.popStack = new Stack<>();
}
/** Push element x to the back of queue. */
public void push(int x) {
this.pushStack.push(x);
}
/** Removes the element from in front of queue and returns that element. */
public int pop() {
this.reloadPop();
return this.popStack.pop();
}
/** Get the front element. */
public int peek() {
this.reloadPop();
return this.popStack.peek();
}
/** Returns whether the queue is empty. */
public boolean empty() {
return this.pushStack.empty() && this.popStack.empty();
}
private void reloadPop() {
if (this.popStack.size() == 0) {
while (this.pushStack.size() > 0) {
this.popStack.push(this.pushStack.pop());
}
}
}
}
