Reverse Polish notation evaluator

Evaluate an expression given in Reverse Polish Notation (postfix). The input is a list of tokens where each token is either an integer (already an int) or one of the operator strings '+', '-', '*', '/'. Apply each operator to the two most recently produced operands, where for a binary operator the earlier-pushed operand is the left operand. Division truncates toward zero (like C). The expression is always valid; assume no division by zero. The token list has at least one token. Return the integer result.