input: Write a python function to find the pair of numbers in an array whose sum is closest to zero.
  assert find_sum_close_to_zero([1, 60, -10, 70, -80, 85]) == -10
  assert find_sum_close_to_zero([10, 22, 28, 29, 30, 40]) == 52
  assert find_sum_close_to_zero([-21, -67, -37, -18, 4, -65]) == -14
output: def find_sum_close_to_zero(arr):
      arr.sort()
      n = len(arr)
      left = 0
      right = n - 1
      min_sum = float('inf')
      closest_pair = (0, 0)
  
      while left < right:
          curr_sum = arr[left] + arr[right]
          
          if abs(curr_sum) < abs(min_sum):
              min_sum = curr_sum
              closest_pair = (arr[left], arr[right])
          
          if curr_sum < 0:
              left += 1
          else:
              right -= 1
  
      return closest_pair[0] + closest_pair[1]