ImplementQueueusingStacks.java

package easy;

import java.util.Stack;
/**
 * 
 * ClassName: ImplementQueueusingStacks
 * @author chenyiAlone  
 * Create Time: 2019/02/11 20:03:28
 * Description: No.232
 * 
 * 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.
    Example:
    
    MyQueue queue = new MyQueue();
    
    queue.push(1);
    queue.push(2);  
    queue.peek();  // returns 1
    queue.pop();   // returns 1
    queue.empty(); // returns 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).
 */
public class ImplementQueueusingStacks {
    Stack<Integer> stack;
    /** Initialize your data structure here. */
    public ImplementQueueusingStacks() {
        stack = new Stack<>();
    }
    
    /** Push element x to the back of queue. */
    public void push(int x) {
        int[] tmp = new int[stack.size()];
        for (int i = stack.size() - 1; i >= 0; i--) {
            tmp[i] = stack.pop();
        }
        stack.push(x);
        for (int i : tmp) {
            stack.push(i);
        }
        
    }
    
    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        return stack.pop();
    }
    
    /** Get the front element. */
    public int peek() {
        return stack.peek();
    }
    
    /** Returns whether the queue is empty. */
    public boolean empty() {
        return stack.isEmpty();
    }
}

/**
 * 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();
 */