leetcode232. Implement Queue using Stacks

232. Implement Queue using Stacks

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).

解法

当push操作时，存入第一个栈中。当进行peek和pop操作时，检测stack2是否为空，如果为空，从第一个栈中pop数据，push到stack2中。如果不为空，直接peek和pop。如果stack1和stack2都为空，则结果为空。

public class MyQueue {
    Stack<Integer> stack1;
    Stack<Integer> stack2;
    /** Initialize your data structure here. */
    public MyQueue() {
        stack1 = new Stack<Integer>();
        stack2 = new Stack<Integer>();
    }

    /** Push element x to the back of queue. */
    public void push(int x) {
        stack1.push(x);
    }

    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        peek();
        if (!stack2.empty()) {
            return stack2.pop();
        }
        return -1;
    }

    /** Get the front element. */
    public int peek() {
        if (stack2.empty()) {
            while (!stack1.empty()) {
                stack2.push(stack1.pop());
            }
        }
        if (!stack2.empty()) {
            return stack2.peek();
        }
        return -1;
    }

    /** Returns whether the queue is empty. */
    public boolean empty() {
        return stack2.empty() && stack1.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();
 */