自己实现一个栈，要求这个栈具有push()、pop()（返回栈顶元素并出栈）、peek() （返回栈顶元素不出栈）、isEmpty()、size()这些基本的方法。

提示：每次入栈之前先判断栈的容量是否够用，如果不够用就用Arrays.copyOf()进行扩容；

  public class MyStack {
  private int[] storage;//存放栈中元素的数组
  private int capacity;//栈的容量
  private int count;//栈中元素数量
  private static final int GROW_FACTOR = 2;

  //TODO：不带初始容量的构造方法。默认容量为8
  public MyStack() {
    this.capacity = 8;
    this.storage=new int[8];
    this.count = 0;
  }

  //TODO：带初始容量的构造方法
  public MyStack(int initialCapacity) {
    if (initialCapacity < 1)
      throw new IllegalArgumentException("Capacity too small.");
    this.capacity = initialCapacity;
    this.storage = new int[initialCapacity];
    this.count = 0;
  }

  //TODO：入栈
  public void push(int value) {
    if (count == capacity) {
      ensureCapacity();
    }
    storage[count++] = value;
  }

  //TODO：确保容量大小
  private void ensureCapacity() {
    int newCapacity = capacity * GROW_FACTOR;
    storage = Arrays.copyOf(storage, newCapacity);
    capacity = newCapacity;
  }

  //TODO：返回栈顶元素并出栈
  private int pop() {
    count--;
    if (count == -1)
      throw new IllegalArgumentException("Stack is empty.");
    return storage[count];
  }

  //TODO：返回栈顶元素不出栈
  private int peek() {
    if (count == 0){
      throw new IllegalArgumentException("Stack is empty.");
    }else {
      return storage[count-1];
    }
  }
  //TODO：判断栈是否为空
  private boolean isEmpty() {
    return count == 0;
  }
  
  //TODO：返回栈中元素的个数
  private int size() {
    return count;
  }

验证

MyStack myStack = new MyStack(3);
myStack.push(1);
myStack.push(2);
myStack.push(3);
myStack.push(4);
myStack.push(5);
myStack.push(6);
myStack.push(7);
myStack.push(8);
System.out.println(myStack.peek());//8
System.out.println(myStack.size());//8
for (int i = 0; i < 8; i++) {
    System.out.println(myStack.pop());
}
System.out.println(myStack.isEmpty());//true
myStack.pop();//报错：java.lang.IllegalArgumentException: Stack is empty.