var search= new List<string> {"s1","s2","s3","s4","s5"};
var rep=new List<string> {"N1","n2","n3","n4","n5"};
string text=" sss s1 sss s2 dsfs s3 sdsdf s5";
search.ForEach(s=>
{
text=text.Replace(s,rep.ElementAt(search.IndexOf(s))+"\n");
}
)
;
return text...