creating parallel processing of parenthesis balancer

I am attending coursera scala course for parallel programming. I have an assignment to solve paranthesis balancer both sequential and parallel. I have solved the sequential function. For the parallel function I have doubts as to how I can maintain the current paranthesis data across parallel threads.

Here is my sequential function:

def balance(chars: Array[Char]): Boolean = {

    def helper( arr: Array[Char], acc: Int): Boolean = {
      if (arr.isEmpty && acc ==0)
        true
      else if (arr.isEmpty  || arr.head == ')' && acc <=0)
        false
      else if (arr.head == '(')
        helper( arr.tail,  acc +1)
      else if (arr.head == ')' )
        helper( arr.tail,  acc - 1)
      else
        helper(arr.tail, acc)
    }

    helper(chars, 0)
  }

now, my whole logic is based on acc value. When I will run these across multiple threads, what should I be focusing on? The parallel method will have range start and end index. The only thing I have understood so far is that aggregate of all these operations should be acc==0.

Please let me know some pointers in this direction.

Thanks