15. 3Sum

Given an integer array nums, return all the triplets [nums[i], nums[j], nums[k]] such that i != j, i != k, and j != k, and nums[i] + nums[j] + nums[k] == 0.

Notice that the solution set must not contain duplicate triplets.

def threeSum(nums):
       
    def twoSum(start, target):
        lo, hi = start, L - 1        
        while (lo < hi):
            s = nums[lo] + nums[hi]
            if s < target:
                lo += 1
            elif s > target:
                hi -= 1
            else:
                yield [nums[lo], nums[hi]]
                lo += 1
                hi -= 1
                while lo < hi and nums[lo] == nums[lo - 1]:
                    lo += 1

    res = []
    nums.sort()
    L = len(nums)
   
    for i in range(L):
        if nums[i] > 0:
            break
        if i == 0 or nums[i - 1] != nums[i]:
            for pair in twoSum(i+1, -nums[i]):
                res.append([nums[i]] + pair)
           
    return res

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