Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.

• push(x) -- Push element x onto stack.
• pop() -- Removes the element on top of the stack.
• top() -- Get the top element.
• getMin() -- Retrieve the minimum element in the stack.

class MinStack {
    private Stack<Integer> num;
    private Stack<Integer> min;

    /** initialize your data structure here. */
    public MinStack() {
        num = new Stack<>();
        min = new Stack<>();
    }

    public void push(int x) {
        num.push(x);
        int m = this.min.size() == 0 ? x : Math.min(x, min.peek());
        min.push(m);
    }

    public void pop() {
        num.pop();
        min.pop();
    }

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

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

/**
 * Your MinStack object will be instantiated and called as such:
 * MinStack obj = new MinStack();
 * obj.push(x);
 * obj.pop();
 * int param_3 = obj.top();
 * int param_4 = obj.getMin();
 */