设计一个具有getMin()功能的栈

/**
* 设计一个具有getMin()功能的栈：
* 1.pop，push，getMin()时间复杂度为O(1)
* 2.设计的栈可以使用现有的栈结构
* @author Nemo
*
*/
public class StackDemo {

private  Stack<Integer> stackData = new Stack<Integer>();
private  Stack<Integer> stackMin = new Stack<Integer>();
public void push(Integer data){
if(data == null){
throw new IllegalArgumentException("Illegal argument");
}
if(stackMin.isEmpty() || stackMin.peek() >=data){
this.stackMin.push(data);
}else{
this.stackMin.push(stackMin.peek());
}
this.stackData.push(data);
}

public Integer pop(){
if(stackData.isEmpty()){
throw new RuntimeException("stack is empty");
}
this.stackMin.pop();
return this.stackData.pop();
}

public Integer getMin(){
if(stackMin.isEmpty()){
throw new RuntimeException("stack is empty");
}
return stackMin.peek();
}