hw2

F is the `frontier' queue
N is the `next' set
U is the 'unvisited' set

Push:
U = all vertices
F = one vertex v0
remove v0 from U //mark v0 as visited
While U is nonempty
    For each v in F //key difference: only check F
        For each child u of v //key difference: if several parents of u belong to F, we need to check u several times 
            If u is in U 
                remove u from U //mark u as visited 
                add u to N
            endIf
        endFor
    endFor
    F = N //`next' becomes `frontier'
endWhile

Pull:
U = all vertices
F = one vertex v0
remove v0 from U //mark v0 as visited
While U is nonempty
    For each vertex u in U //key difference: all vertices are checked
        If at least one parent of u is in F //key difference: if several parents of u belong to F, we only access u once 
            remove u from U //mark u as visited 
            add u to N
        endIf
    endFor
    F = N //`next' becomes `frontier' 
endWhile

reference: Yang et al., Implementing Push-Pull E€iciently in GraphBLAS, ICPP 2018.