Stack • Applications

Evaluate Postfix Expression

Postfix expressions (Reverse Polish Notation) can be evaluated cleanly in a single left-to-right pass using a Stack. No parentheses or precedence rules are needed!

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The Logic

Read the expression from left to right. If you see a number (operand), push it to the stack. If you see an operator (+, -, *, /), it acts on the two most recent numbers.

Pop Order Matters

When an operator is found, pop TWO numbers. The first pop is the right operand (val2). The second pop is the left operand (val1). Then calculate `val1 OPERATOR val2`.

The Result

Push the calculated result back onto the stack. By the end of the expression, exactly one number will remain in the stack: the final answer.

231*+9-

Empty Stack

Code Tracker

:::

1function evaluatePostfix(exp) {

2 let stack = [];

3 for (let token of exp) {

4 if (!isNaN(token)) {

5 stack.push(Number(token));

6 } else {

7 let val2 = stack.pop();

8 let val1 = stack.pop();

9 if (token === '+') stack.push(val1 + val2);

10 else if (token === '-') stack.push(val1 - val2);

11 else if (token === '*') stack.push(val1 * val2);

12 else if (token === '/') stack.push(Math.floor(val1 / val2));

13 }

14 }

15 return stack.pop();

16}