力扣-15-三数之和

问题：

# 给你一个包含 n 个整数的数组 nums，判断 nums 中是否存在三个元素 a，b，c ，使得 a + b + c = 0 ？请你找出所有和为 0 且不重
# 复的三元组。 
# 
#  注意：答案中不可以包含重复的三元组。 

方法一：BF（submit超时）

# leetcode submit region begin(Prohibit modification and deletion)
class Solution(object):
    def threeSum(self, nums):
        """
        :type nums: List[int]
        :rtype: List[List[int]]
        """
        nums.sort()
        res = []
        length = len(nums)
        for i in range(length-2):
            if nums[i] != nums[i-1] or i == 0:
                for j in range(i+1, length-1):
                    if nums[j] != nums[j-1] or j==i+1:
                        for k in range(j+1, length):
                            if nums[k] != nums[k-1] or k==j+1:
                                if nums[i] + nums[j] + nums[k] == 0:
                                    res.append([nums[i], nums[j], nums[k]])
        return res
# leetcode submit region end(Prohibit modification and deletion)

方法二：排序+双指针(左右夹逼)

# leetcode submit region begin(Prohibit modification and deletion)
class Solution(object):
    def threeSum(self, nums):
        """
        :type nums: List[int]
        :rtype: List[List[int]]
        """
        nums.sort()
        res = []
        length = len(nums)
        for i in range(length):
            if i > 0 and nums[i] == nums[i-1]: continue
            left, right = i + 1, length-1
            while left < right:
                v = nums[i] + nums[left] + nums[right]
                if v < 0:
                    left += 1
                elif v > 0:
                    right -= 1
                else:
                    res.append([nums[i], nums[left], nums[right]])
                    while left < right and nums[left] == nums[left+1]: left+=1
                    while left < right and nums[right] == nums[right-1]: right-=1
                    left += 1
                    right -= 1
        return res
# leetcode submit region end(Prohibit modification and deletion)