tree-balancing

I am reading a book on data structures and it says that a left balanced binary tree is a tree in which the leaves only occupy the leftmost positions in the last level. This seemed a little vague to me. Does this mean that leaves are only on the left side of a root and are distributed throughout the whole level, or leave exists only on the left side of the entire tree. Exactly what constitute left balanced? I am not sure if my guess even covers any of the answer, so if anyone could help, it would be greatly appreciated :-). You can think of a left-balanced binary tree as a balanced binary tree

I'm looking for the best way to calculate a nodes balance in an AVL-tree . I thought I had it working, but after some heavy inserting/updating I can see that it's not working correct (at all). This is kind of a two-part question, the first part would be how to calculate the height of a sub-tree, I know the definition "The height of a node is the length of the longest downward path to a leaf from that node." and I understand it, but I fail at implementing it. And to confuse me further this quote can be found on wikipedia on tree-heights "Conventionally, the value -1 corresponds to a subtree