Leetcode-15 三数之和

class Solution(object):
    def threeSum(self, nums):
        """
        :type nums: List[int]
        :rtype: List[List[int]]
        """
        nums.sort()
        res, k = [], 0
        for k in range(len(nums) - 2):
            if nums[k] > 0: break # 1. because of j > i > k.
            if k > 0 and nums[k] == nums[k - 1]: continue # 2. skip the same `nums[k]`.
            i, j = k + 1, len(nums) - 1
            while i < j: # 3. double pointer
                s = nums[k] + nums[i] + nums[j]
                if s < 0:
                    i += 1
                    while i < j and nums[i] == nums[i - 1]: i += 1
                elif s > 0:
                    j -= 1
                    while i < j and nums[j] == nums[j + 1]: j -= 1
                else:
                    res.append([nums[k], nums[i], nums[j]])
                    i += 1
                    j -= 1
                    while i < j and nums[i] == nums[i - 1]: i += 1
                    while i < j and nums[j] == nums[j + 1]: j -= 1
        return res

class Solution:
    def threeSum(self, nums: List[int]) -> List[List[int]]:

        if len(nums) < 3: return []
        nums.sort()
        ans, firstNum_set, ans_set = [], set(), set()

        for i in range(len(nums) - 2):
            base = nums[i]
            if base not in firstNum_set:
                firstNum_set.add(base)
                target = 0 - base
                left, right = i + 1, len(nums) - 1

                if nums[left] + nums[left + 1] > target or nums[right] + nums[right - 1] < target: # 剪枝
                    continue
                
                while left < right: # 双指针
                    if nums[left] + nums[right] == target:
                        if (base, nums[left], nums[right]) not in ans_set:
                            ans.append([base, nums[left], nums[right]])
                            ans_set.add((base, nums[left], nums[right]))
                        left += 1
                        right -= 1
                    elif nums[left] + nums[right] > target:
                        right -= 1
                    else:
                        left += 1
        
        return ans