def two_sum_fast(lst):
"""
Count the number of pair of numbers add up to zero. first sort the list,
then use binary_search the get the other number which could add up to zero,
if in the list, then increase the counter.
>>> lst = [-1, 1, -2, 3, 5, -5, 0, 4]
>>> two_sum_fast(lst)
2
"""
lst.sort()
cnt = 0
for i in range(len(lst)):
if binary_search(-lst[i], lst) > i:
cnt += 1
return cnt
def three_sum_fast(lst):
"""
Count how many three numbers add up to zero. first sort the list,
then using two for-loop and binary search algorithm get the opposite number.
>>> lst = [-1, 2, 1, 3, 0, 4, -4, 5, 9, -5]
>>> three_sum_fast(lst)
8
"""
lst.sort()
cnt = 0
for i in range(len(lst)):
for j in range(i + 1, len(lst)):
if binary_search(-lst[i] - lst[j], lst) > j:
cnt += 1
return cnt
