232. Implement Queue using Stacks

Leetcode Stack Design

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).

```Java class MyQueue { private int[] items; private int size; private int head; private int tail; private int len;

/** Initialize your data structure here. */
public MyQueue() {
    items = new int[10];
    size = 0;
    head = 0;
    tail = 0;
    len = 0;
}

/** Push element x to the back of queue. */
public void push(int x) {
    items[tail++] = x;
    if (tail == size) {
        resize(2*size);
    }
    len++;
}

/** Removes the element from in front of queue and returns that element. */
public int pop() {
    len--;
    return items[head++];
}

/** Get the front element. */
public int peek() {
    return items[head];
}

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

private void resize(int newSize) {
    int newItems[] = new int[newSize];
    for (int i = head, j = 0; i <= tail; i++, j++) {
        newItems[i] = items[j];
    }
    head = 0;
    tail = items.length;
    size = newSize;
    items = newItems;
}

}

/* * 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(); / ```