[CC150][3.2] How would you design a stack which, in addition to push and pop, also has a function min which returns the minimum element? Push, pop and min should all operate in O(1) time.

minStack.java

import java.util.Stack;
import java.util.Random;

public class minStack<T extends Comparable<? super T>> {
	Stack<T> data;
	Stack<T> min;
	T minimum;
	int N;

	public minStack() {
		data = new Stack<T>();
		min = new Stack<T>();
		N = 0;
	}
	
	public void push(T item) {
		data.add(item);
		if (N == 0)
			minimum = item;
		else if (item.compareTo(minimum) < 0)
			minimum = item;
		min.add(minimum);
		N++;
	}
	
	public T pop() {
		if (N == 0) return null;
		N--;
		min.pop();
		return data.pop();
	}
	
	public T min() {
		if (N == 0) return null;
		return min.peek();
	}
	
	public String toString() {
		return data.toString();
	}
	
	public static void main(String[] args) {
		minStack<Double> test = new minStack<Double>();
		Random rd = new Random();
		for (int i = 0; i < 5; i++)
			test.push(rd.nextDouble());
		System.out.println(test.toString() + " min: " + test.min());
		test.pop();
		System.out.println(test.toString() + " min: " + test.min());
		test.pop();
		System.out.println(test.toString() + " min: " + test.min());
		test.pop();
		System.out.println(test.toString() + " min: " + test.min());
		test.pop();
		System.out.println(test.toString() + " min: " + test.min());
		test.pop();
		System.out.println(test.toString() + " min: " + test.min());
	}
}