刷 leetecode 的时候发现同样一段代码，用 Python 通过了，用 py3 就超时，这段代码有什么问题吗？

rt 附上代码，求大神解答。
class Solution:
# @return a list of lists of length 3, [[val1,val2,val3]]
def threeSum(self, num):
num.sort()
res = []
for i in range(len(num)-2):
if i == 0 or num[i] > num[i-1]:
left = i + 1
right = len(num) - 1
while left < right:
if num[left] + num[right] == -num[i]:
res.append([num[i], num[left], num[right]])
left += 1
right -= 1
while left < right and num[left] == num[left-1]:
left += 1
while left < right and num[right] == num[right+1]:
right -= 1
elif num[left] + num[right] < -num[i]:
while left < right:
left += 1
if num[left] > num[left-1]:
break
else:
while left < right:
right -= 1
if num[right] < num[right+1]:
break
return res