package com.free;

public class Stack<E>  {
    private Object[] data = null;

    private int maxSize = 0; // 栈的容量

    private int top = -1; // 栈顶的指针

    // 构造函数：根据指定的size初始化栈

    Stack() {
        this(10);
    }

    Stack(int initialSize) {
        if (initialSize >= 0) {
            this.maxSize = initialSize;
            this.data = new Object[initialSize];
            this.top = -1;
        } else {
            throw new RuntimeException("初始化大小不能为0:" + initialSize);
        }
    }

    /**
     * 向栈中压入一个数据，先入栈的数据在最下面
     *
     *
     * @param e
     * @return
     */
    public boolean push(E e) {
        if (top == maxSize - 1) {
            throw new RuntimeException("栈已满，无法将元素入栈");
        } else {
            data[++top] = e;
            return true;
        }
    }

    /**
     * 弹出栈顶数据，既移除栈顶数据
     * @return
     */
    public E pop() {
        if (top == -1) {
            throw new RuntimeException("栈为空！");
        } else {
            return (E) data[top--];
        }
    }

    /**
     * 返回当前的栈顶数据
     * @return
     */
    public E peek() {
        if (top == -1) {
            throw new RuntimeException("栈为空！");
        }

        return (E) data[top];
    }
}