Balancing Brackets Algorithm: Stack Data Structure

Question

I am currently refining my understanding of data structures and as such, decided to implement a popular application of the Stack. Below you will find my implementation of the Balancing Symbols algorithm written in JavaScript.

My basic questions are:

How clean would you rate my code?
Is time complexity O(n): Linear?
How can I improve?

function isBalanced(arr){
  var stack = new Stack();
  var last;

  arr.forEach(function(curr){
    if (curr === '(' || curr === '[' || curr === '{') {
      stack.push(curr);
    }

    if (curr === ')' || curr === ']' || curr === '}') {
      last = stack.peek() + curr;
      if (last === '()' || last === '[]' || last === '{}') {
        stack.pop();
      }
    }
  });

  var result = (stack.length() === 0) ?  true : false;
  console.log(result);
}

var symbols = ['{', '(', ')', ']'];

isBalanced(symbols);

200_success · Accepted Answer · 2016-09-18 04:21:02Z

2

The code looks reasonably clean, IMO. I assume that it is purposely ES5?

I don't know about Stack as it's not a ES5 standard API (that I know of).

You are not returning any value. There is no need for ? true : false — just do return stack.length() === 0;.

Some code that you can consider, in ES5.

var completes = ['()', '[]', '{}'];
var open = completes.map(function(complete) {
  return complete.charAt(0);
});
var close = completes.map(function(complete) {
  return complete.charAt(1);
});

function isBalanced(arr) {
  var stack = [];
  arr.forEach(function(curr) {
    if (open.indexOf(curr) !== -1) {
      stack.push(curr);
    }
    if (close.indexOf(curr) !== -1) {
      var last = stack.pop();
      if (completes.indexOf(last + curr) === -1) {
        stack.push(last);
      }
    }
  });
  return stack.length === 0;
}

console.log(isBalanced(['{', '(', ')', ']']));
console.log(isBalanced(['[', '(', ')', ']']));

edited Sep 18, 2016 at 4:21

200_success

146k22 gold badges191 silver badges481 bronze badges

answered Sep 17, 2016 at 23:43

Xotic750

6356 silver badges15 bronze badges

\$\begingroup\$ Yes it is ES5 on purpose, I just wanted to get the concepts down rather than config my environment to execute ES6. Stack was a class that I created to mimic the functionality from other OOP languages. \$\endgroup\$

Pazuzu
– Pazuzu

2016-09-18 00:41:17 +00:00
Commented Sep 18, 2016 at 0:41
\$\begingroup\$ I assume that Stack is pretty similar in functionality as the code in my example? \$\endgroup\$

Xotic750
– Xotic750

2016-09-18 00:50:36 +00:00
Commented Sep 18, 2016 at 0:50
\$\begingroup\$ Yes, arrays in JS have almost identical functionality to the methods in my Stack class. You're solid. \$\endgroup\$

Pazuzu
– Pazuzu

2016-09-18 00:52:44 +00:00
Commented Sep 18, 2016 at 0:52

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