r - How to concatenate strings in a specified order

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Tried to concatenate strings
diagonally from this post href="https://stackoverflow.com/questions/24346355/how-to-alternatively-concatenate-3-strings">how
to alternatively concatenate 3 strings, but was not
successful.

My input
is:

a<-c("a1","a2","a3")
b<-c("b1","b2","b3")
c<-c("c1","c2","c3")

My
expected output would be

 "a1"
            "b2" "c3" "a2" "b3"
            "a3"

How to get the
above from

 c(rbind(a,b,c))
            

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class="normal">Answer

How about
ordering the vector by values derived by the row and columns after setting the lower
diagonal to missing

mat <-
            rbind(a,b,c)

mat[lower.tri(mat)] <-
            NA
na.omit(mat[order(col(mat) -
            row(mat))])