code: function( node, level ) {
              node.leftNeighborId = this.lastNodeOnLevel[level];
              if ( node.leftNeighborId ) {
                  node.leftNeighbor().rightNeighborId = node.id;
              }
              this.lastNodeOnLevel[level] = node.id;
              return this;
          }
docs: Algorithm works from left to right, so previous processed node will be left neighbour of the next node
  @param {TreeNode} node
  @param {number} level
  @returns {Tree}