Three sum problem in python. Find three numbers from an array whose sum is x

three_sum.py

'''
three sum, to find a+b+c== x in an array 
'''
a= [-1,0,1,2,-1,-4] #the array/ list
x= 0  #the sum we need

s= set()
res= [] #holds groups of 3 numbers satisfying the condition
for i in range(len(a)-1):
  ax= a[i]
  l, r= i+1, len(a)-1
  while l < r:
    if x -(ax+ a[l]) in s:
      res.append([ax, a[l], x -(ax+ a[l])])
    else:
      s.add(ax+ a[l])
    l+=1
  s.clear()
print(res)  #[[-1, 2, -1], [0, -1, 1]]