c++ - Reconcile two lists -

so have 2 different lists, different formats , structure need reconciled. essentially, set b needs match what's in set a, want preserve state of existing items in set b , not overwrite them what's in set a.

for reference, list doesn't mean list. "lists" come in couple of different forms straight arrays maps. use standard iterators access elements.

the way typically handle so...

for item in lista   if listb contains item      mark item in list b visited   else       add item list b   item in listb    if visited true       continue    else        add item removelist   item in removelist     remove item list b 

this works , real way can think of it. don't how many iterations have though, having 3 loops back feels wrong. however, since i'm using iterators can't remove lists while i'm checking them , instead have add them third remove list.

in potential answers please keep in mind speed , memory footprint more important me how easy write code.

my question boils down -- there better way i'm not thinking of?

i'm in c++/c fwiw, though think solution language agnostic.

thanks!

here's way may more efficient:

removelist = listb  item in lista   if listb contains item     remove item removelist   else     add item listb  item in removelist   remove item listb 

so instead of building removelist nothing, starts off , has items removed it.

you make more efficient having removelist store indexes rather actual items. long items added end of listb in initial loop, , items removed in reverse order, indexes should still valid.

in fact it's simpler if replace removelist boolean array of items keep. algorithm becomes this:

initialise itemstokeep false savedlistlength = length of listb  item in lista   offset = find item in listb   if found     mark itemstokeep[offset] true   else     add item listb  offset savedlistlength-1 down 0   if itemstokeep[offset] false     remove offset listb 

this avoids need copy removelist initially. , expense of itemstokeep array surely no worse whatever using keep track of visted items in algorithm.

to extent suitable algorithm maybe depend on form of lists (i.e. vectors, or linked lists, etc.) think approach has potential more effecient either way.