Counting pais with difference

Counting pairs.py

def sort(array):
    """Sort the array by using quicksort."""

    less = []
    equal = []
    greater = []

    if len(array) > 1:
        pivot = array[0]
        for x in array:
            if x < pivot:
                less.append(x)
            elif x == pivot:
                equal.append(x)
            elif x > pivot:
                greater.append(x)
    
        # Removing non-unique elememts
        m = [pivot]
   
        return sort(less)+m+sort(greater)  

    else:  
        return array

      
# A sorting based program to  
# count pairs with difference k 

def countPairsWithDiffK(numbers,k): 
    count =0
    
    # Sort array elements 
    arr = sort(numbers)  
    # pointer left
    l =0
    # pointer right
    r=0

 
    while r<(len(arr)): 
        if arr[r]-arr[l]==k: 
            count+=1
            l+=1
            r+=1

        elif arr[r]-arr[l]>k:  
            l+=1
        else: 
            r+=1

    return count