假设：

• 级别按从底部顶点开始的最短路径的长度分组。
• 从上到下索引。
• 底部顶点没有任何向外的边。

library(igraph)
    
    ## create some DAG.
    g <- graph_from_literal(a -+ b:c:d:e, b -+ d, c -+ d:e, d -+ e)
    g <- set_edge_attr(g, name="weight", value=-1)          # longest path = shortest negative.
    dist   <- (-distances(g, v=(V(g)), mode="out"))         # matrix of VxV distances of shortest paths.
    layers <- apply(dist, 1, max)                           # maximum distance from bottom to top (max per row).
    layers <- max(layers) - layers                          # minimal distance from top    to bottom.
    
    plot(g, layout=layout_with_sugiyama(g, layers=layers))  # show the graph by level.
      
    d = 1                                                   # certain depth d. 
    layers[which(layers==d)]
    ## b c 
    ## 1 1

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更详细的答案，请参阅：igraph：树图，其中终端（而不是根）节点在同一级别？