GFG problem Find common elements in three sorted arrays

Question

For this problem my approach is : copy all the elements of all three arrays in vector. Then count the frequency of each element in the vector. Any element which have frequency equal to 3 ,will be printed.

MY CODE:

class Solution
{
    public:    
       vector <int> commonElements (int A[], int B[], int C[], int n1, 
       int n2, int n3)
        {
            //code here.
            vector<int>v;
            unordered_map<int, int> mp;
            //copying the elements of all the array in the vector
            for(int i=0;i<n1;i++){
                v.push_back(A[i]);
            }
            for(int i=0;i<n2;i++){
                v.push_back(B[i]);
            }
            for(int i=0;i<n3;i++){
                v.push_back(C[i]);
            }
            
 
    // Traverse through array elements and
    // count frequencies 
    
    for (int i = 0; i < v.size(); i++)
        mp[v[i]]++;
 
    // Traverse through map and print frequencies
    for (auto x : mp){
        if(x.second==3){
            cout<<x.first;
        }
    }
    
    
        }

};

In this solution, I am getting a segmentation fault. But I am unable to understand why I am getting this error. And also can anyone please explain the meaning of line->

vector <int> commonElements (int A[], int B[], int C[], int n1, 
       int n2, int n3)