#150 - Evaluate Reverse Polish Notation

Problem Description

You are given an array of strings tokens that represents an arithmetic expression in a Reverse Polish Notation.

Evaluate the expression. Return an integer that represents the value of the expression.

Note that:

The valid operators are '+', '-', '*', and '/'.
Each operand may be an integer or another expression.
The division between two integers always truncates toward zero.
There will not be any division by zero.
The input represents a valid arithmetic expression in a reverse polish notation.
The answer and all the intermediate calculations can be represented in a 32-bit integer.

Solution

/**
 * @param {string[]} tokens
 * @return {number}
 */
var evalRPN = function(tokens) {
  const stack = [];
  const operators = {
    '+': (a, b) => a + b,
    '-': (a, b) => a - b,
    '*': (a, b) => a * b,
    '/': (a, b) => a / b >= 0 ? Math.floor(a / b) : Math.ceil(a / b),
  };

  tokens.forEach(token => {
    if (operators[token]) {
      const item = stack.pop();
      stack.push(operators[token](stack.pop(), item));
    } else {
      stack.push(Number(token));
    }
  });

  return stack.pop();
};