Leetcode•Jul 25, 2025

Evaluate Reverse Polish Notation

Hazrat Ali

Leetcode

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.

 

Example 1:

Input: tokens = ["2","1","+","3","*"]
Output: 9
Explanation: ((2 + 1) * 3) = 9

Example 2:

Input: tokens = ["4","13","5","/","+"]
Output: 6
Explanation: (4 + (13 / 5)) = 6

Example 3:

Input: tokens = ["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

Solution

/**
 * @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;
};



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