Divide And Conquer Sum Of Array Of Integers

Question

I wrote the below program to find the sum of all integers using divide and conquer recursion algorithm in Java:

But somehow the sum is coming incorrectly.

public class DivideAndConquerSum {
 public static void main(String[] args) {
    int[] arr = new int[]{2, 3, 4, 5};
    System.out.println(calculateRecursiveSum(arr, 0, arr.length));
  }
  static long sum = 0;
  static long calculateRecursiveSum(int[] arr, int low, int high) {
    if (high == low) {
      return arr[0];
    } else {
      int mid = (high + low) / 2;
      sum = calculateRecursiveSum(arr, low, mid) +
            calculateRecursiveSum(arr, mid + 1, high);
    }
    return sum;
  }
}

Can anyone please let me know what is wrong in the code to resolve it? Assuming only positive integers in this scenario.

Answer 1

Your method basically recalculates mid so you need to return the value at that point. It is more suited to a binary search. But make the following changes and it will work.

       static long calculateRecursiveSum(int[] arr, int low, int high) {
          if (high == low) {
                return 0;
            }
            int mid = (high + low) / 2;
            return arr[mid] + calculateRecursiveSum(arr, low, mid) + 
                      calculateRecursiveSum(arr, mid+1, high);
       }

Answer 2

Your approach was almost okay, here I corrected few problems to keep your idea working.

public class DivideAndConquerSum
{
   public static void main(String[] args)
   {
      int[] arr = new int[] { 2, 3, 4, 5 };
      System.out.println(calculateRecursiveSum(arr, 0, arr.length - 1));
   }

   // static long sum = 0;
   static long calculateRecursiveSum(int[] arr, int low, int high)
   {
      // long sum=0;
      if (high == low)
      {
         return arr[low];
      }
      else
      {
         int mid = (high + low) / 2;
         return (calculateRecursiveSum(arr, low, mid) + calculateRecursiveSum(arr, mid + 1, high));
      }
      // return sum;
   }
}

You don't need an extra static variable to store the result. Returning the recursive call directly will do it for you.