feat: add rust codes for chapter computational complexity (#714)

codes/rust/chapter_tree/binary_search_tree.rs

@@ -76,7 +76,7 @@ impl BinarySearchTree {
     pub fn insert(&mut self, num: i32) {
         // 若树为空，则初始化根节点
         if self.root.is_none() {
-            self.root = TreeNode::new(num);
+            self.root = Some(TreeNode::new(num));
             return;
         }
         let mut cur = self.root.clone();