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leetcode_18 4Sum

2024-09-22 18:33:49   212人阅读

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

Note: The solution set must not contain duplicate quadruplets.

For example, given array S = [1, 0, -1, 0, -2, 2], and target = 0.

A solution set is:
[
  [-1,  0, 0, 1],
  [-2, -1, 1, 2],
  [-2,  0, 0, 2]
]
找到数列中所有和等于目标数的四元组，需去重.多枚举一个数后，参照3Sum的做法，O(N^3)  

python实现
class Solution(object):
    def fourSum(self, nums, target):
        nums.sort()
        res = []
        length = len(nums)
        for i in range(0, length - 3):
            if i and nums[i] == nums[i - 1]:
                continue
            for j in range(i + 1, length - 2):
                if j != i + 1 and nums[j] == nums[j - 1]:
                    continue
                sum = target - nums[i] - nums[j]
                left, right = j + 1, length - 1
                while left < right:
                    if nums[left] + nums[right] == sum:
                        res.append([nums[i], nums[j], nums[left], nums[right]])
                        right -= 1
                        left += 1
                        while left < right and nums[left] == nums[left - 1]:
                            left += 1
                        while left < right and nums[right] == nums[right + 1]:
                            right -= 1
                    elif nums[left] + nums[right] > sum:
                        right -= 1
                    else:
                        left += 1
        return res

 

 

leetcode_18 4Sum


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