public int getMin() {
      if (isEmpty()) {
        return Integer.MAX_VALUE;
      } else {
        return peek().minValue;
      }
    }
Approach based on a inner data structure to store the stack of integers and in the same object
the current min value we had when the element was pushed. With this approach we get a time
complexity order equals to O(1) but a space complexity order equals to O(N) where N is the
number of elements in the stack.
src/main/java/com/github/pedrovgs/problem63/ConstantComplexityOrderStack.java