0150-evaluate-reverse-polish-notation.js

/**
 * 150. Evaluate Reverse Polish Notation
 * https://leetcode.com/problems/evaluate-reverse-polish-notation/
 * Difficulty: Medium
 *
 * 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.
 */

/**
 * @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();
};