实现一个栈，可以使用除了栈之外的数据结构
eg:
输入：
push(1)
pop()
push(2)
top() // return 2
pop()
isEmpty() // return true
push(3)
isEmpty() // return false

个人实现：

public class Stack {
    private int initCapacity = 8;
    private Integer top = null;
    private int curIndex = 0;
    private Integer[] stack = new Integer[initCapacity];
    /*
     * @param x: An integer
     * @return: nothing
     */
    public void push(int t) {
        // write your code here
         int length = stack.length;
        if (curIndex == length) {
            resizeCapacity();
        }
        top = t;
        stack[curIndex++] = t;
    }
 // 扩容
    private void resizeCapacity() {
        int length = stack.length;
        if (length >= Integer.MAX_VALUE) {
            throw new RuntimeException("the stack max empty is" + Integer.MAX_VALUE);
        }
        Integer[] newStack = new Integer[length << 1];
        System.arraycopy(stack, 0, newStack, curIndex-1, length);
        stack = newStack;
    }
    /*
     * @return: nothing
     */
    public void pop() {
        // write your code here
         if(top==null){
            return;
        }
        stack[--curIndex]=null;
        if(curIndex==0){
            top = null;
        }else{
            top = stack[curIndex-1];
        }
        
    }

    /*
     * @return: An integer
     */
    public int top() {
        // write your code here
      
        return top;
    }

    /*
     * @return: True if the stack is empty
     */
    public boolean isEmpty() {
        // write your code here
        return top == null;
    }
}