Stack.java

package com.github.chishiwu.coding2017.basic;

public class Stack {
	 private Node mStackNode;
	    private int size;

	    public void push(Object o) {
	        Node node = new Node();
	        node.data = o;
	        if (null == mStackNode) {
	            mStackNode = node;
	        } else {
	            mStackNode.next = node;
	            mStackNode = node;
	        }
	        size++;
	    }

	    public Object pop() {
	        if (size == 0) {
	            throw new RuntimeException("the stack is empty");
	        }
	        Object obj = mStackNode.data;
	        mStackNode = mStackNode.pre;
	        size--;
	        return obj;
	    }

	    public Object peek() {
	        if (size == 0) {
	            throw new RuntimeException("the stack is empty");
	        }
	        return mStackNode.data;
	    }

	    public boolean isEmpty() {
	        return size == 0;
	    }

	    public int size() {
	        return size;
	    }

	    private static class Node {
	        Object data;
	        Node next;
	        Node pre;
	    }
}