diff --git a/ch4/hierholzer.ml b/ch4/hierholzer.ml
new file mode 100644
index 0000000..1bb892a
--- /dev/null
+++ b/ch4/hierholzer.ml
@@ -0,0 +1,33 @@
+open Scanf
+open Printf
+
+type graph = int array array (* Adjacency List representation *)
+
+let hierholzer (adj : graph) (start : int) : int list =
+  let n = Array.length adj in
+  let idx = Array.make n 0 in
+  let rec iter path = function
+    | [] -> path
+    | u :: stack' as stack ->
+      if idx.(u) < Array.length adj.(u) then begin
+        let v = adj.(u).(idx.(u)) in
+        idx.(u) <- idx.(u) + 1;
+        iter path (v :: stack)
+      end else
+        iter (u :: path) stack' in
+  iter [] [start]
+
+
+let () =
+  let adj = [|
+    [1; 6];
+    [2];
+    [3; 4];
+    [0];
+    [5];
+    [0; 2];
+    [5];
+  |] |> Array.map Array.of_list in
+  let answer = hierholzer adj 0 in
+  answer |> List.iter (fun u -> printf "%c " (char_of_int (int_of_char 'A' + u)));
+  printf "\n"