Counting pairs with range sum in a limit in JavaScript

Range Sum

Range sum rangeSum(i, j) is defined as the sum of the elements in an array between indices i and j (i ≤ j), inclusive.

Problem

We are required to write a JavaScript function that takes in an array of Integers, arr, as the first argument and two numbers, upper and lower as the second and third element.

Our function is supposed to return the number of range sums that lie between the range [upper, lower], (both inclusive).

For example, if the input to the function is −

const arr = [1, 4, 3];
const upper = 5;
const lower = 2;

Then the output should be −

const output = 3;

Example

The code for this will be −

const arr = [1, 4, 3];
const upper = 5;
const lower = 2;
const countRangeSum = (arr = [], lower, upper) => {
   const sums = [0];
   let res = 0;
   let last = 0;
   let firstge = value => {
      let l = 0, r = sums.length, m;
      do {
         m = Math.floor((r + l) / 2);
         sums[m] < value ? l = m : r = m;
      } while (r >= l + 2);
      while (r > 0 && sums[r - 1] >= value ) {
         r -= 1;
      }
      return r;
   };
   arr.forEach(num => {
      last += num;
      res += firstge(last - lower + 1) - firstge(last - upper);
      sums.splice(firstge(last), 0, last);
   });
   return res;
};
console.log(countRangeSum(arr, lower, upper));

Output

The output in the console will be −

3