[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>> ret;
        
        sort(nums.begin(), nums.end());
        
        // for i and j, need to avoid duplicates.
        for (int i = 0; i < nums.size(); i++){
            if (i > 0 && nums[i] == nums[i-1]) continue;
            for (int j = i + 1; j < nums.size(); j++){
                // here j >  ....
                if (j > i + 1 && nums[j] == nums[j-1]) continue;
                int left = j + 1;
                int right = nums.size() - 1;
                while (left < right){
                    int tmp = nums[i] + nums[j] + nums[left] + nums[right];
                    if (tmp < target){
                        left++;
                    }else if (tmp > target){
                        right--;
                    }else{
                        vector<int> sol;
                        sol.push_back(nums[i]);
                        sol.push_back(nums[j]);
                        sol.push_back(nums[left]);
                        sol.push_back(nums[right]);
                        ret.push_back(sol);
                        left++;
                        right--;
                        while(nums[left] == nums[left-1]) left++;
                        while(nums[right] == nums[right+1]) right--;
                    }
                }    
            }
        }
        
        return ret;
    }
};