Why is this game over function always returning false?

I have made a function which should check if a 2048 game is in a game over situation. This is when there are no zeros and no two adjacent numbers that are equal. When in a game over situation it should return zero and otherwise should be false.

The issue is that the code here always returns false and I do not understand why this is. I would appreciate any help:

bool game_over(const std::vector<int>& v){

    int length = v.size();

    for (int i = 0; i < length; i++){
         if (v[i] == 0){
             return false;
         }
    }

    for( int i= 0; i < length ; i++ ){
        for( int j = 0; j < length; j++){

            int x = v[twod_to_oned(i, j, length)];

            if (i < length  && x == v[twod_to_oned(i+1, j, length )]){
                return false;
            }

            if (j < length  && x == v[twod_to_oned(i, j+1, length)]){
                return false;
            }   
        }
    }
    return true;
}
int twod_to_oned(int row, int col, int rowlen){
    return row*rowlen+col;
}

The test I used:

int main(){

    std::vector<int> v{1,1,1,1};

    bool c = game_over(v);

    if(c){
        std::cout << "true" << std::endl;
    }

    if(!c){
        std::cout << "false" << std::endl;
    }

    return 0;
}