class Solution {public:    vector
    
     
      > threeSum(vector
      
       & nums) {        set
       
        
         > res;        sort(nums.begin(), nums.end());        if (nums.empty() || nums.back() < 0 || nums.front() > 0) return {};        for (int k = 0; k < nums.size(); ++k) {            if (nums[k] > 0) break;            int target = 0 - nums[k];            int i = k + 1, j = nums.size() - 1;            while (i < j) {                if (nums[i] + nums[j] == target) {                    res.insert({nums[k], nums[i], nums[j]});                    while (i < j && nums[i] == nums[i + 1]) ++i;                    while (i < j && nums[j] == nums[j - 1]) --j;                    ++i; --j;                } else if (nums[i] + nums[j] < target) ++i;                else --j;            }        }        return vector
         
          
           >(res.begin(), res.end()); }};