自定义栈返回最小值

自定义栈 O(1)时间内返回最小值？

public class MinStack {

Stack<Integer> dataStack = new Stack<>();
Stack<Integer> assistStack = new Stack<>();

public void push(int node) {
    dataStack.push(node);
    if(assistStack.isEmpty() || node < assistStack.peek()){
        assistStack.push(node);
    }else {
        assistStack.push(assistStack.peek());//保证此位置出栈后能正确返回最小值
    }
}

public void pop() {
    if(!dataStack.isEmpty() && !assistStack.isEmpty()){
        dataStack.pop();
        assistStack.pop();
    }
}

public int top() {
    if(dataStack.isEmpty())
        throw new RuntimeException("error: stack is empty");

    return dataStack.peek();
}

public int min() {
    if(assistStack.isEmpty()){
        throw new RuntimeException("error: stack is empty");
    }
    return assistStack.peek();
}
}