100170. Implement Queue using Stacks LCCI

难度: Easy

题目描述

Implement a MyQueue class which implements a queue using two stacks.

 

Example:

MyQueue queue = new MyQueue();

queue.push(1);
queue.push(2);
queue.peek();  // return 1
queue.pop();   // return 1
queue.empty(); // return false

 

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).

 

解题思路

代码实现

解决方案

java

class MyQueue {

    private Stack<Integer> data;
    private Stack<Integer> temp;

    private int top=0;

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

    /** Push element x to the back of queue. */
    public void push(int x) {
        if (top == 0) {
            top = x;
        }
        data.push(x);
    }

    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        if (data.size() == 0) {
            return 0;
        }
        if (data.size() == 1) {
            top = 0;
            return data.pop();
        }
        while (data.size() > 1) {
            if (data.size() == 2) {
                top = data.peek();
            }
            temp.push(data.pop());
        }
        int res = data.pop();
        while (temp.size() > 0) {
            data.push(temp.pop());
        }
        return res;
    }

    /** Get the front element. */
    public int peek() {

        return top;

    }

    /** Returns whether the queue is empty. */
    public boolean empty() {
            return data.size()==0;
    }
}

/**
 * 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();
 */