toString() {
    let string = [];

    for (var [data, node] of this.nodes.entries()) {
      const adjacetsData = node.adjacents.map((adj) => adj.data).join(', ');
      if(adjacetsData){
        string.push(`${data}: ${adjacetsData}`);
      }
    }

    return string.join('\n');
  }
Return adjacency matrix of the data
@returns {string}