Add function to find leafs.

.gitignore

@@ -1 +1,2 @@
 /target
+Cargo.lock

src/lib.rs

@@ -81,6 +81,21 @@ impl<NodeValueT: NodeValue> Graph<NodeValueT> {
         // This is always 0.
         0
     }
+
+    /// Find all nodes that are leaf nodes.
+    /// A leaf node is one that doesn't have any children.
+    pub fn find_leaf_keys(&self) -> Result<Vec<Key>, GraphError> {
+        let mut leaf_keys = Vec::<Key>::new();
+
+        for key in 0..self.nodes.len() {
+            let children_of_node = self.children_of(&key)?;
+            if children_of_node.len() == 0 {
+                leaf_keys.push(key);
+            }
+        }
+
+        return Ok(leaf_keys);
+    }
 }
 
 #[derive(Debug, Clone)]
@@ -145,4 +160,25 @@ mod tests {
         assert!(graph.parent_of(&1).unwrap() == Some(0));
         assert!(graph.children_of(&1).unwrap().len() == 0);
     }
+
+    #[test]
+    fn graph_search() {
+        let root_val = NodeType::new(1.0);
+        let mut graph = Graph::new(root_val);
+
+        let second_val = NodeType::new(2.0);
+        let second_add_res = graph.add(second_val, 0);
+        assert!(second_add_res.is_ok());
+
+        let third_val = NodeType::new(3.0);
+        let third_add_res = graph.add(third_val, 0);
+        assert!(third_add_res.is_ok());
+
+        let fourth_val = NodeType::new(4.0);
+        let fourth_add_res = graph.add(fourth_val, 1);
+        assert!(fourth_add_res.is_ok());
+
+        let leaf_keys = graph.find_leaf_keys().unwrap();
+        assert!(leaf_keys.len() == 2);
+    }
 }