diff --git a/buggy.py b/buggy.py
--- a/buggy.py
+++ b/buggy.py
@@ -30,7 +30,7 @@
         st = []
         for i in indegree:
             if indegree[i] == 0:
-                st.push(i)
+                st.append(i)
         flst = []
         ans = defaultdict(lambda: [])
         while st:
