evaluate-reverse-polish-notation.js

/**
 * Evaluate Reverse Polish Notation
 *
 * Evaluate the value of an arithmetic expression in Reverse Polish Notation.
 *
 * Valid operators are +, -, *, /. Each operand may be an integer or another expression.
 *
 * Note:
 *
 * Division between two integers should truncate toward zero.
 * The given RPN expression is always valid. That means the expression would always evaluate to a result and there won't
 * be any divide by zero operation.
 *
 * Example 1:
 *
 * Input: ["2", "1", "+", "3", "*"]
 * Output: 9
 *
 * Explanation: ((2 + 1) * 3) = 9
 *
 * Example 2:
 *
 * Input: ["4", "13", "5", "/", "+"]
 * Output: 6
 *
 * Explanation: (4 + (13 / 5)) = 6
 *
 * Example 3:
 *
 * Input: ["10", "6", "9", "3", "+", "-11", "*", "/", "*", "17", "+", "5", "+"]
 * Output: 22
 *
 * Explanation:
 *   ((10 * (6 / ((9 + 3) * -11))) + 17) + 5
 * = ((10 * (6 / (12 * -11))) + 17) + 5
 * = ((10 * (6 / -132)) + 17) + 5
 * = ((10 * 0) + 17) + 5
 * = (0 + 17) + 5
 * = 17 + 5
 * = 22
 */

/**
 * @param {string[]} tokens
 * @return {number}
 */
const evalRPN = tokens => {
  const operands = new Set(['+', '-', '*', '/']);
  const stack = [];

  for (let s of tokens) {
    if (operands.has(s)) {
      const n2 = parseInt(stack.pop());
      const n1 = parseInt(stack.pop());
      stack.push(evaluate(n1, n2, s));
    } else {
      stack.push(s);
    }
  }

  return parseInt(stack.pop());
};

const evaluate = (n1, n2, operand) => {
  if (operand === '+') return n1 + n2;
  if (operand === '-') return n1 - n2;
  if (operand === '*') return n1 * n2;
  if (operand === '/') return n1 / n2;
};

export { evalRPN };