Subtree

For a vertex $u$ in a rooted tree, its subtree contains $u$ and every vertex reachable by repeatedly following children in a rooted tree, together with the connecting edges; its root of a tree is $u$. In a binary tree, the left subtree is rooted at the left child, and is empty when that child is absent; the right subtree is defined similarly. A binary search tree constrains every key in these subtrees.