[LeetCode] 18. 4Sum

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]]

class Solution {public:    vector<vector<int>> fourSum(vector<int>& nums, int target) {        vector<vector<int>> res;        sort(nums.begin(), nums.end());                for (int k1 = 0; k1 < nums.size(); k1++) {            int target3 = target - nums[k1];            for (int k2 = k1 + 1; k2 < nums.size(); k2++) {                int target2 = target3 - nums[k2];                for (int i = k2 + 1, j = nums.size() - 1; i < j; ) {                    int sum2 = nums[i] + nums[j];                    if (sum2 == target2) {                        res.push_back({nums[k1], nums[k2], nums[i], nums[j]});                        for (int dup = nums[i]; i < j && nums[i] == dup; i++);                        for (int dup = nums[j]; i < j && nums[j] == dup; j--);                    } else if (sum2 < target2)                        i++;                    else                        j--;                }                for (int dup = nums[k2]; k2 + 1 < nums.size() && nums[k2 + 1] == dup; k2++);            }            for (int dup = nums[k1]; k1 + 1 < nums.size() && nums[k1 + 1] == dup; k1++);        }                return res;    }};