Array implement stacks and queues

Array stacks:

public class ArrayStack {
    private Integer[] arr;
    private Integer index;

    public ArrayStack(int initSize) {
        if(initSize < 0) {
            throw new IllegalArgumentException("this init size is less than 0");
        }
        arr = new Integer[initSize];
        index = 0;
    }

    public Integer peek() {
        if(index == 0) {
            return null;
        }
        return arr[index - 1];
    }

    public void push(int obj) {
        if(index == arr.length) {
            throw new ArrayIndexOutOfBoundsException("this stack is full");
        }
        arr[index++] = obj;
    }
    public Integer pop() {
        if(index == 0) {
            throw new ArrayIndexOutOfBoundsException("this stack is empty");
        }
        return arr[--index];
    }
    public static void main(String[] args) {
        ArrayStack stack = new ArrayStack(5);
        stack.push(5);
        stack.push(3);
        System.out.println(stack.pop());
        stack.pop();
        System.out.println(stack.peek());

    }

}

The result:

an array queue:

public class ArrayQueue {
    private Integer[] arr;
    private Integer size;
    private Integer first;
    private Integer last;

    public ArrayQueue(int initSize) {
        if(initSize == 0) {
            throw new IllegalArgumentException("this is queue is less than 0");
        }
        arr = new Integer[initSize];
        size = 0;
        first = 0;
        last = 0;
    }

    public Integer peek() {
        if(size == 0) {
            return null;
        }
        return arr[first];
    }

    public void push(int obj) {
        if(size == arr.length) {
            throw new ArrayIndexOutOfBoundsException("this queue is full");
        }
        size++;
        arr[last] = obj;
        last = last == arr.length - 1 ? 0 : last + 1; 
    }

    public Integer poll() {
        if(size == 0) {
            throw new ArrayIndexOutOfBoundsException("this queue is empty");
        }
        size--;
        int tmp = first;
        first = first == arr.length - 1 ? 0 : first + 1;
        return arr[tmp];
    }
    public static void main(String[] args) {
        ArrayQueue queue = new ArrayQueue(5);
        queue.push(4);
        queue.push(6);
        System.out.println(queue.poll());
        System.out.println(queue.peek());
    }

}

operation result:

Take the smallest element of the stack: the stack to achieve a particular, realization of the basic functions on the stack, and then the operation returns to achieve getMin smallest elements in the stack. Requirements are as follows:

pop、push、getMin操作的时间复杂度都是O(1)。
设计的栈类型可以使用现成的栈结构。
思路:使用两个栈,stack2栈顶元素维持stack1中最小元素的值,当入栈的元素小于stack2栈顶元素时直接入satck2,否则取栈顶元素的值再入satck2.

import java.util.Stack;

public class work3 {
    private Stack<Integer> stack1;
    private Stack<Integer> stack2;
    public work3() {
        this.stack1 = new Stack<Integer>();
        this.stack2 = new Stack<Integer>();
    }
    public void push(int num) {
        if(this.stack2.isEmpty()) {
            this.stack2.push(num);
        }else if(num < this.getMin()) {
            this.stack2.push(num);
        }else {
            int newNum = this.stack2.peek();
            this.stack2.push(newNum);
        }
        this.stack1.push(num);
    }
    public int pop() {
        if(this.stack1.isEmpty()) {
            throw new RuntimeException("You stack is empty.");
        }
        this.stack2.pop();
        return this.stack1.pop();
    }
    public int getMin() {
        if(this.stack2.isEmpty()) {
            throw new RuntimeException("You stack is empty.");
        }
            return this.stack2.peek();
    }
    public static void main(String[] args) {
        work3 stack = new work3();
        stack.push(5);
        System.out.println(stack.getMin());
        stack.push(1);
        System.out.println(stack.getMin());
        stack.pop();
        stack.push(2);
        System.out.println(stack.getMin());
    }

}

operation result: