18+fourSum.cpp

class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
        vector<vector<int>> res;
        std::sort(nums.begin(), nums.end());
        int i, j, left, right;

        for (i = 0; i < nums.size(); i++) {
            if (i > 0 && nums[i] == nums[i - 1]) {
                continue;
            }
            for (j = i + 1; j < nums.size(); j++) {
                if (j > i + 1 && nums[j] == nums[j - 1]) {
                    continue;
                }
                left = j + 1;
                right = nums.size() - 1;

                while (left < right) {
                    if (nums[i] + nums[j] + nums[left] + nums[right] < target) {
                        left++;
                    } else if (nums[i] + nums[j] + nums[left] + nums[right] > target) {
                        right--;
                    } else if (nums[i] + nums[j] + nums[left] + nums[right] == target) {
                        res.push_back(vector<int>{nums[i], nums[j], nums[left], nums[right]});
                        
                        left++;
                        right--;

                        while (left < right && nums[left] == nums[left - 1]) {
                            left++;
                        }
                        
                        while (left < right && nums[right] == nums[right + 1]) {
                            right--;
                        }
                    }
                }
            }
        }

        return res;
    }
};