private int getRootHeight(BinaryNode tree) {
    if (tree == null) {
      return 0;
    } else {
      return 1 + Math.max(getRootHeight(tree.getLeft()), getRootHeight(tree.getRight()));
    }
  }
Recursive algorithm to check if a binary tree is balanced. The complexity order in time terms
of this algorithm is equals to O(N) where N is the number of nodes in the binary tree. In
space terms, the complexity order is O(1) because we are not using any auxiliary data
structure to resolve this problem. This algorithm is based on the tree height because the
balanced tree definition is based on this.
src/main/java/com/github/pedrovgs/problem65/IsTreeBalanced.java