This one uses the same way like other posts to avoid duplicates, but beats 99% of python submissions. The idea is to skip some iterations when the sum is obviously positive.

Here are the details (assume i<j<k):

(1) a[i] >0, stop iteration.

(2) For every j, record the smallest k in the list kref. This value is used as the max limit for k of iteration i+1, since sum(i+1, j, k) > sum(i, j, k) > 0.

```
if len(nums) < 3:
return []
nums = sorted(nums)
kref = [len(nums)-1 for _ in nums]
out = []
i = 0
while nums[i] <= 0 and i < len(nums)-2:
if i > 0 and nums[i] == nums[i-1]:
i += 1
continue
j = i+1
k = kref[j]
while j < k:
s = nums[i]+nums[j]+nums[k]
if s < 0:
j += 1
elif s > 0:
k -= 1
kref[j] = k
else:
out.append([nums[i], nums[j], nums[k]])
while j < k and nums[j] == nums[j+1]:
j += 1
while j < k and nums[k] == nums[k-1]:
k -= 1
j += 1
k -= 1
i += 1
return out
```