Evaluate a Postfix Expression

In postfix notation the operator comes after its two operands, so 3 4 + means 3 + 4. A stack evaluates it in one pass. The Deque, the split into tokens, the loop and isOperator() are already written.

Requirements:
1. For a number token, push its int value onto the stack.
2. For an operator token, pop two values and push the result of +, -, * or / on them. The first pop is the right operand and the second pop is the left one, which matters for - and /.
3. Use integer division for /, and return the single value left on the stack, or -1 if there is none.

Do NOT write a main method.