flatten.d

struct Node(T) {
   T val;
   Node!T[] children;
}

import std.range;
ForwardRange!T flatten(T)(Node!T root) {
   import std.algorithm;
   return only(root.val).chain(map!flatten(root.children).joiner).inputRangeObject;
}

void main() {
   alias N = Node!int;
   auto a = N(1, [
      N(2, [
         N(3, [
            N(4)
         ])
      ]),
      N(5)
   ]);

   import std.stdio;
   writeln(a.flatten);
}