Evaluate a +/- expression tree

Problem A binary tree encodes an arithmetic expression: internal nodes are '+' or '-' operators, leaves are numeric values. Compute the value at the root.

Input / Output

• Input: root of the expression tree.
• Output: evaluated integer.

Constraints

• Tree depth may be large — discuss recursion limits; operators are binary (exactly two children).

Example

• (+ (- 5 2) 3) → 6.