15. 3Sum

題目:
https://leetcode.com/problems/3sum/

難度:

Medium

第一想法，先把nums排序，用三個loop，無法AC

class Solution(object):
    def threeSum(self, nums):
        """
        :type nums: List[int]
        :rtype: List[List[int]]
        """
        n = len(nums)
        res = []
        nums.sort()
        for i in range(n):
            for j in range(i,n):
                for k in range(j,n):
                    if nums[i] + nums[j] + nums[k] == 0 and j != i and k != j and k != i: 
                        curRes = [nums[i],nums[j],nums[k]]
                        if curRes not in res:
                            res.append(curRes)
    
        return res

然后查了一下2sum，用2sum的花樣，因為要排除重復以及輸出是按照從小到大的輸出:

class Solution(object):
    def threeSum(self, nums):
        """
        :type nums: List[int]
        :rtype: List[List[int]]
        """
        n = len(nums)
        nums.sort()
        self.res = []
        for i in range(n):
            self.twoSum(nums[i+1:], 0-nums[i])
        return [list(i) for i in self.res]
        
    def twoSum(self, nums, target):
        """
        :type nums: List[int]
        :type target: int
        :rtype: List[int]
        """
        lookup = {}
        for num in nums:
            if target - num in lookup:
                if (-target ,target - num, num) not in self.res:
                    self.res.append((-target ,target - num, num))
            lookup[num] = target - num