Problem50 - done

switch to own leftist tree impl
Problem54A - done (trivial)
Problem55 - done
Problem56 - done
Problem57 - done
Problem58 - done
Problem58 - simplify a little bit
Problem59 - done
Problem60 - minNodes and maxHeight
Problem60 - done
Problem61 - done
Problem61A - done
Problem62 - done
Problem62B - done
Problem63 - generate complete bin tree
Problem63 - done
Problem64 - done
trees are foldable
tree has depth
tree - handling empty tree case