Arithmetic Ancestor Tree

The Codetikki elders encode arithmetic expressions as binary trees: each leaf is an integer, and each internal node is one of +, -, *, or /. Given the root of such a tree, evaluate it. Input: a binary tree in level-order, with operators as single characters (+, -, *, /) and leaves as integers. Use "null" for absent nodes. Output: the integer result of evaluating the expression (use integer division for /).