Add ability to get keys in a depth first order.

src/lib.rs

@@ -96,6 +96,26 @@ impl<NodeValueT: NodeValue> Graph<NodeValueT> {
 
         return Ok(leaf_keys);
     }
+
+    /// Return all keys ordered like you were doing
+    /// a depth first search.
+    pub fn get_keys_by_depth(&self) -> Result<Vec<Key>, GraphError> {
+        let mut visited_keys = Vec::<Key>::new();
+        let mut stack = Vec::<Key>::new();
+
+        stack.push(self.root_key());
+
+        while stack.len() > 0 {
+            let next_key = stack.pop().unwrap();
+            if !visited_keys.contains(&next_key) {
+                visited_keys.push(next_key);
+                let children_of_next_key = self.children_of(&next_key)?;
+                stack.extend(children_of_next_key);
+            }
+        }
+
+        Ok(visited_keys)
+    }
 }
 
 #[derive(Debug, Clone)]
@@ -178,7 +198,15 @@ mod tests {
         let fourth_add_res = graph.add(fourth_val, 1);
         assert!(fourth_add_res.is_ok());
 
+        let fifth_val = NodeType::new(5.0);
+        let fifth_add_res = graph.add(fifth_val, 2);
+        assert!(fifth_add_res.is_ok());
+
         let leaf_keys = graph.find_leaf_keys().unwrap();
         assert!(leaf_keys.len() == 2);
+
+        let keys_by_depth = graph.get_keys_by_depth().unwrap();
+        let expected_keys_by_depth = vec![0, 2, 4, 1, 3];
+        assert!(keys_by_depth == expected_keys_by_depth);
     }
 }