Evaluate Reverse Polish Notation

mediumcodingAlgorithmsStacksMath~30 min

Asked at Google, Meta

Overview

Evaluate the value of an arithmetic expression in Reverse Polish Notation (postfix). Valid operators are +, -, *, /. Division truncates toward zero.

Constraints

1 <= tokens.length <= 10,000
Each token is an operator or an integer in the range [-200, 200].
The expression is always valid.

Examples

evalRPN(["2","1","+","3","*"]);
// => 9  ((2+1)*3)

evalRPN(["4","13","5","/","+"]);
// => 6  (4+(13/5))

Solution

Reveal solution

function evalRPN(tokens) {
  const stack = [];
  for (const t of tokens) {
    if ("+-*/".includes(t)) {
      const b = stack.pop(), a = stack.pop();
      if (t === "+") stack.push(a + b);
      else if (t === "-") stack.push(a - b);
      else if (t === "*") stack.push(a * b);
      else stack.push(Math.trunc(a / b));
    } else {
      stack.push(parseInt(t));
    }
  }
  return stack[0];
}

Resources

Evaluate Reverse Polish Notation — LeetCode

evaluate-reverse-polish-notation.js

Evaluate Reverse Polish Notation

mediumcodingAlgorithmsStacks

Overview

Evaluate the value of an arithmetic expression in Reverse Polish Notation (postfix). Valid operators are +, -, *, /. Division truncates toward zero.

Constraints

1 <= tokens.length <= 10,000
Each token is an operator or an integer in the range [-200, 200].
The expression is always valid.

Examples

evalRPN(["2","1","+","3","*"]);
// => 9  ((2+1)*3)

evalRPN(["4","13","5","/","+"]);
// => 6  (4+(13/5))

Solution

Reveal solution

function evalRPN(tokens) {
  const stack = [];
  for (const t of tokens) {
    if ("+-*/".includes(t)) {
      const b = stack.pop(), a = stack.pop();
      if (t === "+") stack.push(a + b);
      else if (t === "-") stack.push(a - b);
      else if (t === "*") stack.push(a * b);
      else stack.push(Math.trunc(a / b));
    } else {
      stack.push(parseInt(t));
    }
  }
  return stack[0];
}

Resources

Evaluate Reverse Polish Notation — LeetCode

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