4Sum

    vector<vector<int> > fourSum(vector<int> &num, int target) {
        vector<vector<int> > ret;
        if(num.size()<4)
            return ret;
        sort(num.begin(),num.end());
        for(int i=0;i<num.size()-3;i++){
            if(i>0&&num[i]==num[i-1]) continue;
            for(int j=i+1;j<num.size()-2;j++){
                if(j>i+1&&num[j]==num[j-1]) continue;
                int l=j+1,r=num.size()-1;
                while(l<r){
                    int sum=num[i]+num[j]+num[l]+num[r];
                    if(sum>target)
                        r--;
                    else if(sum<target)
                        l++;
                    else{
                        vector<int> tmp;
                        tmp.push_back(num[i]);tmp.push_back(num[j]);tmp.push_back(num[l]);tmp.push_back(num[r]);
                        ret.push_back(tmp);
                        do{l++;}while(l<r&&num[l]==num[l-1]);
                        do{r--;}while(l<r&&num[r]==num[r+1]);
                    }                       
                }

            }
        }
        return ret;
    }