4Sum

Problem

Given an array nums of n integers and an integer target, are there elements a, b, c, and d in nums such that a + b + c + d = target? Find all unique quadruplets in the array which gives the sum of target.

Notice that the solution set must not contain duplicate quadruplets.

For example:

Input: nums = [1,0,-1,0,-2,2], target = 0
Output: [[-2,-1,1,2],[-2,0,0,2],[-1,0,0,1]]

Solution

class Solution:
    def fourSum(self, nums: List[int], target: int) -> List[List[int]]:	
        soln = []
        nums = sorted(nums)
        
        for i in range(len(nums)-3):
            if i!=0 and nums[i] == nums[i-1]:
                continue
            for j in range(i+1, len(nums)-2):
                if j != i+1 and nums[j] == nums[j-1]:
                    continue
                left = j+1
                right = len(nums)-1
                
                while left < right:
                    currentSum = nums[i]+nums[j]+nums[left]+nums[right]
                    if currentSum < target:
                        left+=1
                    elif currentSum > target:
                        right-=1
                    else:
                        soln.append([nums[i], nums[j], nums[left], nums[right]])
                        left+=1
                        right-=1
                
                        while left < right and nums[left] == nums[left-1]:
                            left+=1
                        
                        while left < right and nums[right] == nums[right+1]:
                            right-=1
                    
        return soln
                    

#Time: O(n^3)
#Space: O(1)