README_EN.md

18. 4Sum

中文文档

Description

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.

Note:

The solution set must not contain duplicate quadruplets.

Example:

Given array nums = [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]

]

Solutions

Python3

class Solution:
    def fourSum(self, nums: List[int], target: int) -> List[List[int]]:
        res = []
        if nums is None or len(nums) < 4:
            return res
        n = len(nums)
        nums.sort()
        for i in range(n - 3):
            if i > 0 and nums[i] == nums[i - 1]:
                continue
            for j in range(i + 1, n - 2):
                if j > i + 1 and nums[j] == nums[j - 1]:
                    continue
                p, q = j + 1, n - 1
                while p < q:
                    if p > j + 1 and nums[p] == nums[p - 1]:
                        p += 1
                        continue
                    if q < n - 1 and nums[q] == nums[q + 1]:
                        q -= 1
                        continue
                    t = nums[i] + nums[j] + nums[p] + nums[q]
                    if t == target:
                        res.append([nums[i], nums[j], nums[p], nums[q]])
                        p += 1
                        q -= 1
                    elif t < target:
                        p += 1
                    else:
                        q -= 1
        return res

Java

class Solution {
    public List<List<Integer>> fourSum(int[] nums, int target) {
        int n;
        if (nums == null || (n = (nums.length)) < 4) {
            return Collections.emptyList();
        }
        Arrays.sort(nums);
        List<List<Integer>> res = new ArrayList<>();
        for (int i = 0; i < n - 3; ++i) {
            if (i > 0 && nums[i] == nums[i - 1]) {
                continue;
            }
            for (int j = i + 1; j < n - 2; ++j) {
                if (j > i + 1 && nums[j] == nums[j - 1]) {
                    continue;
                }
                int p = j + 1, q = n - 1;
                while (p < q) {
                    if (p > j + 1 && nums[p] == nums[p - 1]) {
                        ++p;
                        continue;
                    }
                    if (q < n - 1 && nums[q] == nums[q + 1]) {
                        --q;
                        continue;
                    }
                    int t = nums[i] + nums[j] + nums[p] + nums[q];
                    if (t == target) {
                        res.add(Arrays.asList(nums[i], nums[j], nums[p], nums[q]));
                        ++p;
                        --q;
                    } else if (t < target) {
                        ++p;
                    } else {
                        --q;
                    }
                }
            }
        }
        return res;
    }
}

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