Rename the naming of the coding files

in backtracking algorithm.
Add the typedef to docs.

codes/java/chapter_backtracking/preorder_traversal_i_compact.java

@@ -0,0 +1,44 @@
+/**
+ * File: preorder_traversal_i_compact.java
+ * Created Time: 2023-04-16
+ * Author: Krahets (krahets@163.com)
+ */
+
+package chapter_backtracking;
+
+import include.*;
+import java.util.*;
+
+public class preorder_traversal_i_compact {
+    static List<TreeNode> res;
+
+    /* 前序遍历：例题一 */
+    static void preOrder(TreeNode root) {
+        if (root == null) {
+            return;
+        }
+        if (root.val == 7) {
+            // 记录解
+            res.add(root);
+        }
+        preOrder(root.left);
+        preOrder(root.right);
+    }
+
+    public static void main(String[] args) {
+        TreeNode root = TreeNode.listToTree(Arrays.asList(1, 7, 3, 4, 5, 6, 7));
+        System.out.println("\n初始化二叉树");
+        PrintUtil.printTree(root);
+
+        // 前序遍历
+        res = new ArrayList<>();
+        preOrder(root);
+
+        System.out.println("\n输出所有值为 7 的节点");
+        List<Integer> vals = new ArrayList<>();
+        for (TreeNode node : res) {
+            vals.add(node.val);
+        }
+        System.out.println(vals);
+    }
+}