leetcode 15. 3Sum

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Given an array S of n integers, are there elements a, b, c in S such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.

Note: The solution set must not contain duplicate triplets.

For example, given array S = [-1, 0, 1, 2, -1, -4],

A solution set is:
[
[-1, 0, 1],
[-1, -1, 2]
]

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class Solution(object):
    def threeSum(self, nums):
        """
        :type nums: List[int]
        :rtype: List[List[int]]
        """
        nums.sort()
        res = []
        for i in range(len(nums) - 2):
            if i > 0 and nums[i] == nums[i-1]:
                continue
            l,r = i + 1,len(nums)-1
            while l < r:
                s = nums[i] + nums[r] + nums[l]
                if s > 0:
                    r -= 1
                elif s < 0:
                    l += 1
                else:
                    res.append([nums[i],nums[l],nums[r]])
                    while l < r and nums[l] == nums[l + 1]:
                        l += 1
                    while l < r and nums[r] == nums[r - 1]:
                        r -= 1
                    l += 1
                    r -= 1
        return res