public int getMin() {
      if (innerStack.isEmpty()) {
        return Integer.MAX_VALUE;
      } else {
        return innerStack.peek();
      }
    }
Approach based on keep a parallel data structure with the min elements. Doing this we can
reduce the number of additional elements stored, but the complexity order is still O(N) in
space terms for the worst case. In time terms, the complexity order is equals to O(1).
src/main/java/com/github/pedrovgs/problem63/ConstantComplexityOrderStack.java