Min Stack

Use two stacks to maintain min value.

class MinStack {
    Stack<Integer> s1 = new Stack();
    Stack<Integer> s2 = new Stack();

    public void push(int x) {
        if (x <= getMin()) {
            s2.push(x);
        }
        s1.push(x);
    }

    public void pop() {
        int value = s1.pop();
        if (value == getMin()) {
            s2.pop();
        }
    }

    public int top() {
        return s1.peek();
    }

    public int getMin() {
        if (s2.isEmpty()) {
            return Integer.MAX_VALUE;
        } else {
            return s2.peek();
        }
    }
}