在树中查找上一个节点

Question

我有以下代码：

df <- read.table(text='verkoop          V621  
verkoopcode      V62123  
verkoopcodenaam  V6212355  
verkoopdatum     V621335  
verkoopdatumchar V62133526  
verkooppr        V6216  
verkoopprijs     V62162  
verkoopsafdeling V621213452  
verkoopsartikel  V62126324') 

# use igraph package
require(igraph)

# create adjacency matrix 
adj <- nchar(sapply(df$V1, gsub, x=df$V1, replacement=''))
adj[!sapply(df$V1, grepl, x=df$V1)] <- 0

# name adjecency matrix 
colnames(adj) <- df$V2

# original graph
gr <- graph.adjacency(adj, mode='directed', weighted=TRUE)

layout(matrix(1:2, ncol=2))
plot(gr)

# minimum spanning tree 
mst <- minimum.spanning.tree(gr)
shortest.paths(mst, to="V621", weights=rep(1, ecount(mst)))

现在我为每个节点获取树中的深度。我想确定哪个节点在特定节点之前到来。例如，对于'verkoopdatumchar'，我想找到'verkoopdatum'。