LeetCode 3Sum

Expand the 2Sum solver to 3Sum:

for every specific a, find the other two b&c that b+c=-a.

so, it can be solved in O(n^2).

1. generic "find" function:

iterator = find(vec.begin(), vec.end(), vecElem);

if(iterator == vec.end())　　cout<<"no find the vecElem in vec!"<<endl;

Attach code:

bool compare(const int& a, const int& b)
{
      return (a<b);
}

inline bool nofindit(const vector<vector<int> >& vec, const vector<int> elem)
{
      for(int i=0; i<vec.size(); i++)
      {
            if(vec[i] == elem)
            {
                return false;
            }
      }
      return true;
}

        
class Solution {
public:
    vector<vector<int> > threeSum(vector<int> &num) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
                
        sort(num.begin(), num.end(), compare);
                   
        int len = num.size();
        
        int a,b,c,sum;
        int j;
        int k;
        vector<int> triplet(3,0);
        vector<vector<int> > ret;
        vector<vector<int> >::iterator findit;
    
        for(int i=0; i<=len-3; i++)
        {
            a = num[i];
            j=i+1;
            k=len-1;
            
            while(j<k)
            {
                b = num[j];
                c = num[k];
                sum = a+b+c;
                if( sum  == 0)
                {
                    triplet[0]=a;
                    triplet[1]=b;
                    triplet[2]=c;
                    //findit = find(ret.begin(),ret.end(),triplet);
                    //if(findit==ret.end())                   
                    if( nofindit(ret, triplet) )  
                    {                      
                          ret.push_back(triplet);
                    }
                    j++;k--;
                }
                else if( sum > 0)
                {
                    k--;
                }
                else
                {
                    j++;
                }
            }
        }
        return ret;
        
    }
};