<a target="_blank" href="https://leetcode.com/problems/evaluate-reverse-polish-notation/description/?envType=daily-question&amp;envId=2024-01-30">https://leetcode.com/problems/evaluate-reverse-polish-notation/description/?envType=daily-question&amp;envId=2024-01-30</a>
<pre><code class="lang-javascript">/**
 * @param {string[]} tokens
 * @return {number}
 */
var evalRPN = function (tokens) {
 const operators = {
 "+": (a, b) =&gt; a + b,
 "-": (a, b) =&gt; a - b,
 "*": (a, b) =&gt; a * b,
 "/": (a, b) =&gt; Math.trunc(a / b)
 }

 let stack = []

 for (let token of tokens) {
 if (token in operators) {
 let b = stack.pop()
 let a = stack.pop()
 stack.push(
 operators[token](a, b)
 )
 }
 else {
 stack.push(Number(token))
 }
 }

 return stack[0]

};
</code></pre>
Cool question but remember you have to pop <code>b</code> first before <code>a</code> from stack. Because <code>a</code> are usually the bigger numbers and the reverse polish notation is arranged in the order that makes <code>a</code> bigger.
e.g., ["3", "1", "-"]
Here 3 is at the stack's bottom, so popping it, b would be 1 since it is at top or right and a is 3.

[https://leetcode.com/problems/evaluate-reverse-polish-notation/description/?envType=daily-question&envId=2024-01-30](https://leetcode.com/problems/evaluate-reverse-polish-notation/description/?envType=daily-question&envId=2024-01-30)

```javascript
/**
 * @param {string[]} tokens
 * @return {number}
 */
var evalRPN = function (tokens) {
    const operators = {
        "+": (a, b) => a + b,
        "-": (a, b) => a - b,
        "*": (a, b) => a * b,
        "/": (a, b) => Math.trunc(a / b)
    }

    let stack = []

    for (let token of tokens) {
        if (token in operators) {
            let b = stack.pop()
            let a = stack.pop()
            stack.push(
                operators[token](a, b)
            )
        }
        else {
            stack.push(Number(token))
        }
    }

    return stack[0]

};
```

Cool question but remember you have to pop `b` first before `a` from stack. Because `a` are usually the bigger numbers and the reverse polish notation is arranged in the order that makes `a` bigger.

e.g., \["3", "1", "-"\]

Here 3 is at the stack's bottom, so popping it, b would be 1 since it is at top or right and a is 3.

#150. Evaluate Reverse Polish Notation