MinStack.java

// version 1:
public class MinStack {
    private Stack<Integer> stack;
    private Stack<Integer> minStack;
    
    public MinStack() {
        stack = new Stack<Integer>();
        minStack = new Stack<Integer>();
    }

    public void push(int number) {
        stack.push(number);
        if (minStack.isEmpty()) {
            minStack.push(number);
        } else {
            minStack.push(Math.min(number, minStack.peek()));
        }
    }

    public int pop() {
        minStack.pop();
        return stack.pop();
    }

    public int min() {
        return minStack.peek();
    }
}

// version 2, save more space. but space complexity doesn't change.
public class MinStack {
    private Stack<Integer> stack;
    private Stack<Integer> minStack;

    public MinStack() {
        stack = new Stack<Integer>();
        minStack = new Stack<Integer>();
    }

    public void push(int number) {
        stack.push(number);
        if (minStack.empty() == true)
            minStack.push(number);
        else {
        // 这里考虑的相等的情况也会继续push
        if (minStack.peek() >= number)
            minStack.push(number);
        }
    }

    public int pop() {
        if (stack.peek().equals(minStack.peek()) ) 
            minStack.pop();
        return stack.pop();
    }

    public int min() {
        return minStack.peek();
    }
}