add graphviz src and output

Catoverflow · Catoverflow · commit bee3bf0eccd1 · 2022-06-19T17:13:00.000+08:00
diff --git a/.gitignore b/.gitignore
@@ -7,3 +7,7 @@ lex.yy.c
 .vscode/
 # binary
 ROBDD
+# graphviz source and output
+*.dot
+*.png
+*.svg
