LeetCode 18 [4Sum]

原题

给一个包含n个数的整数数组S，在S中找到所有使得和为给定整数target的四元组(a, b, c, d)。

例如，对于给定的整数数组S=[1, 0, -1, 0, -2, 2]和target=0. 满足要求的四元组集合为：
(-1, 0, 0, 1)
(-2, -1, 1, 2)
(-2, 0, 0, 2)

四元组(a, b, c, d)中，需要满足a <= b <= c <= d
答案中不可以包含重复的四元组

解题思路

• 从2Sum, 3Sum一路演变而来，可以转化为连个for循环+2Sum解决

完整代码

class Solution(object):
    def fourSum(self, nums, target):
        """
        :type nums: List[int]
        :type target: int
        :rtype: List[List[int]]
        """
        res = []
        nums.sort()
        for i in range(len(nums)):
            if i != 0 and nums[i] == nums[i - 1]:
                continue
            
            for j in range(i + 1, len(nums)):
                if j != i + 1 and nums[j] == nums[j - 1]:
                    continue
                
                left, right = j + 1, len(nums) - 1
                while left < right:
                    sum = nums[i] + nums[j] + nums[left] + nums[right]
                    if sum < target:
                        left += 1
                    elif sum > target:
                        right -= 1
                    else:
                        res.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 res