core/4Sum at main · Shreyya18/core · GitHub

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Given an array nums of n integers, return an array of all the unique quadruplets [nums[a], nums[b], nums[c], nums[d]] such that:
0 <= a, b, c, d < n
a, b, c, and d are distinct.
nums[a] + nums[b] + nums[c] + nums[d] == target
You may return the answer in any order.

Example 1:
Input: nums = [1,0,-1,0,-2,2], target = 0
Output: [[-2,-1,1,2],[-2,0,0,2],[-1,0,0,1]]

Example 2:
Input: nums = [2,2,2,2,2], target = 8
Output: [[2,2,2,2]]
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//brute using 4 for loop
class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
        set<vector<int>> st;
        int n=nums.size();
        for(int i=0;i<n-3;i++){
            for(int j=i+1;j<n-2;j++){
                for(int k=j+1;k<n-1;k++){
                    for(int l=k+1;l<n;l++){
                        long long sum=nums[i]+nums[j]+nums[k]+nums[l];
                        if(sum==target){
                            vector<int>temp={nums[i],nums[j],nums[k],nums[l]};
                            sort(temp.begin(),temp.end());
                            st.insert(temp);

                        }
                    }
                }
            }
        }
        vector<vector<int>>ans(st.begin(),st.end());
        return ans;
  }
};
------------------------------------------------------------------------------------------------------------
//better using hashmap
class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
        set<vector<int>> st;
        int n=nums.size();
        for(int i=0;i<n;i++){
            for(int j=i+1;j<n;j++){
                set<long long>hash;
                for(int k=j+1;k<n;k++){
                    long long sum=nums[i]+nums[j]+nums[k];
                    long long fourth=target-sum;
                    if(hash.find(fourth)!=hash.end()){
                        vector<int>temp={nums[i],nums[j],nums[k],(int)(fourth)};
                        sort(temp.begin(),temp.end());
                        st.insert(temp);
                    }
                    hash.insert(nums[k]);
                }
            }
        }
        vector<vector<int>>ans(st.begin(),st.end());
        return ans;
  }
};
----------------------------------------------------------------------------------------------------------
//optimal using 4 pointer
class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
    vector<vector<int>>ans;
        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;
            for(int j=i+1;j<n;j++){
                if(j>i+1 && nums[j]==nums[j-1])continue;

                int k=j+1, l=n-1;
                while(k<l){
                    long long sum=nums[i];
                    sum+=nums[j];
                    sum+=nums[k];
                    sum+=nums[l];
                    if(sum<target){
                        k++;
                    }
                    else if(sum>target){
                        l--;
                    }
                    else{
                        vector<int>temp={nums[i],nums[j],nums[k],nums[l]};
                        ans.push_back(temp);
                        k++;
                        l--;

                        while(k<l && nums[k]==nums[k-1]) k++;
                        while(k<l && nums[l]==nums[l+1]) l--;
                    }
                }

            }
        }
        return ans;

    }
};