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.

3.2 minStack

import java.util.Stack;

@SuppressWarnings("serial")
public class MinStack extends Stack<Integer> {
    private Stack<Integer> minStack;
    
    public MinStack() {
        minStack = new Stack<Integer>();
    }
    
    public void push(int val) {
        super.push(val);
        if (val <= min()) {
            minStack.push(val);
        }
    }
    
    // Must return Integer type.
    public Integer pop() {        
        int val = super.pop();
        if (val == minStack.peek()) {
            minStack.pop();
        }
        
        return val;
    }
    
    public int min() {
        if (super.empty()) {
            return Integer.MAX_VALUE;
        }
        else {
            return minStack.peek();
        }
    }
}