3Sum

Given an integer array nums, return all the triplets [nums[i], nums[j], nums[k]] such that i != j, i != k, and j != k, and nums[i] + nums[j] + nums[k] == 0.
Notice that the solution set must not contain duplicate triplets.

Example 1:
Input: nums = [-1,0,1,2,-1,-4]
Output: [[-1,-1,2],[-1,0,1]]
Explanation: 
nums[0] + nums[1] + nums[2] = (-1) + 0 + 1 = 0.
nums[1] + nums[2] + nums[4] = 0 + 1 + (-1) = 0.
nums[0] + nums[3] + nums[4] = (-1) + 2 + (-1) = 0.
The distinct triplets are [-1,0,1] and [-1,-1,2].
Notice that the order of the output and the order of the triplets does not matter.

Example 2:
Input: nums = [0,1,1]
Output: []
Explanation: The only possible triplet does not sum up to 0.

Example 3:
Input: nums = [0,0,0]
Output: [[0,0,0]]
Explanation: The only possible triplet sums up to 0.
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//brute using 3 for loop
class Solution {
public:
    vector<vector<int>> threeSum(vector<int>& nums) {
        set<vector<int>>set;
       int sum =0;
       int n=nums.size();
       for (int i=0;i<n-2;i++){
        for(int j=i+1;j<n-1;j++){
            for(int k=j+1;k<n;k++){
                sum=nums[i]+nums[j]+nums[k];
                if(sum==0 ){
                    vector<int>temp={nums[i],nums[j],nums[k]};
                    sort(temp.begin(),temp.end());
                    set.insert(temp);
                }
            }
        }
       }
        vector<vector<int>>res(set.begin(),set.end());
       return res;
  }
};
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//better using hashmap
class Solution {
public:
    vector<vector<int>> threeSum(vector<int>& nums) {
       set<vector<int>> st;
        int sum =0 , n=nums.size();
        for(int i=0;i<n-1;i++){
            set<int>hash;
            for(int j=i+1;j<n;j++){
                int third=-(nums[i]+nums[j]);

                if(hash.find(third)!=hash.end()){
                    vector<int>temp={nums[i],nums[j],third};
                    sort(temp.begin(), temp.end());
                    st.insert(temp);
                }
                hash.insert(nums[j]);
            }

        }
        vector<vector<int>>res(st.begin(),st.end());
        return res;
}
};
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//optimal using 3 pointer
class Solution {
public:
    vector<vector<int>> threeSum(vector<int>& nums) {
       vector<vector<int>>res;
        int n=nums.size();
        sort(nums.begin(),nums.end());
        for(int i=0;i<n;i++){
            if(i>0 && nums[i]==nums[i-1]) continue;
            int j=i+1, k=n-1;
            while(j<k){
                int sum=nums[i]+nums[j]+nums[k];
                if(sum<0){
                    j++;
                }
                else if (sum>0){
                    k--;
                }
                else{
                    vector<int>temp={nums[i],nums[j],nums[k]};
                    res.push_back(temp);
                    j++;
                    k--;
                while(j<k && nums[j]==nums[j-1]) j++;
                while(j<k && nums[k]==nums[k+1])k--;
                }
            }
        }
        return res;
    }
};