[Fix] optimized signs

Solves Issue #53

src/PTQ/PTQ.cpp

@@ -699,8 +699,12 @@ std::map<std::string, std::pair<bool, bool>> computeSignMap(std::shared_ptr<Grap
         {
             // Thoses nodes always have a single parent 
             std::shared_ptr<Node> parent = node->getParent(0); 
-            signMap[node->name()].first = signMap[parent->name()].second;
-            signMap[node->name()].second = signMap[node->name()].first;
+            if (parent)
+            {
+                signMap[node->name()].first = signMap[parent->name()].second;
+                signMap[node->name()].second = signMap[node->name()].first;
+            }
+            
         }
     }