18. 四数之和

Description #

Difficulty: 中等

Related Topics: Array, Two Pointers, Sorting

Given an array nums of n integers, return an array of all the unique quadruplets [nums[a], nums[b], nums[c], nums[d]] such that:

• 0 <= a, b, c, d < n
• a, b, c, and d are distinct.
• nums[a] + nums[b] + nums[c] + nums[d] == target

You may return the answer in any order.

Example 1:

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

Example 2:

Input: nums = [2,2,2,2,2], target = 8
Output: [[2,2,2,2]]

Constraints:

• 1 <= nums.length <= 200
• -10⁹ <= nums[i] <= 10⁹
• -10⁹ <= target <= 10⁹

Solution #

Language: Go

定边双指针法：固定两边，指针 i、j 从两边向内计算，如果当前四数之和大于（小于）target，则移动 j（i）想中间靠拢。为了防止超时，需要进行一定的剪枝：当 i、j 确定出一个 target 的解，则需要移动 i、j，移动时需要保证新的 nums[i]、nums[j] 的值是变化了的，否则无论是哪一个值变动都只能让总和更小或者更大，同时也能避免计算了相同解。

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