Thinking process

Get the min?

• using heap, but the pop() is gonna be O(log n)

Using two stacks

• one is regular stack, another only saves current minimum

Mistakes

before using peek(), check whether the stack is empty!!!

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();
    }
}

To improve the code above, save extra space, but space complexity remains the same

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
        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();
    }
}