1.In a full binary tree all nodes have either 0 or 2 children. Both types of nodes can appear at all levels in the tree.
2.In a complete binary tree all levels except the last are completely filled, and in the last level all nodes are to the left as much as possible. This means that all nodes have two children except the nodes at the lowest two levels. At the lowest level the nodes have (by definition) zero children, and at the level above that nodes can have 0, 1 or 2 children.
- perfect binary trees :every node other than the leaves has two children.
