最小栈

leetcode 155
java实现栈： stack类底层是数组，而用linkedlist继承deque底层是链表

	//用栈实现
  	// 数据栈
    private Stack<Integer> data;
    // 辅助栈
    private Stack<Integer> helper;

    /**
     * initialize your data structure here.
     */
    public MinStack() {
        data = new Stack<>();
        helper = new Stack<>();
    }
    public void push(int x) {
        // 数据栈和辅助栈一定会增加元素
        data.add(x);
        if (helper.isEmpty() || helper.peek() >= x) {
            helper.add(x);
        } else {
            helper.add(helper.peek());
        }
    }

    public void pop() {
        // 两个栈都得 pop
        if (!data.isEmpty()) {
            helper.pop();
            data.pop();
        }
    }

    public int top() {
        if(!data.isEmpty()){
            return data.peek();
        }
        throw new RuntimeException("栈中元素为空，此操作非法");
    }

    public int getMin() {
        if(!helper.isEmpty()){
            return helper.peek();
        }
        throw new RuntimeException("栈中元素为空，此操作非法");
    }
}
	//用链表实现栈
    Deque<Integer> xStack;
    Deque<Integer> minStack;

    public MinStack() {
        xStack = new LinkedList<Integer>();
        minStack = new LinkedList<Integer>();
        minStack.push(Integer.MAX_VALUE);
    }
    
    public void push(int x) {
        xStack.push(x);
        minStack.push(Math.min(minStack.peek(), x));
    }
    
    public void pop() {
        xStack.pop();
        minStack.pop();
    }
    
    public int top() {
        return xStack.peek();
    }
    
    public int getMin() {
        return minStack.peek();
    }
}