build

chapter_divide_and_conquer/build_binary_tree_problem.md

@@ -330,9 +330,41 @@ status: new
 === "Dart"
 
     ```dart title="build_tree.dart"
-    [class]{}-[func]{dfs}
+    /* 构建二叉树：分治 */
+    TreeNode? dfs(
+      List<int> preorder,
+      List<int> inorder,
+      Map<int, int> hmap,
+      int i,
+      int l,
+      int r,
+    ) {
+      // 子树区间为空时终止
+      if (r - l < 0) {
+        return null;
+      }
+      // 初始化根节点
+      TreeNode? root = TreeNode(preorder[i]);
+      // 查询 m ，从而划分左右子树
+      int m = hmap[preorder[i]]!;
+      // 子问题：构建左子树
+      root.left = dfs(preorder, inorder, hmap, i + 1, l, m - 1);
+      // 子问题：构建右子树
+      root.right = dfs(preorder, inorder, hmap, i + 1 + m - l, m + 1, r);
+      // 返回根节点
+      return root;
+    }
 
-    [class]{}-[func]{buildTree}
+    /* 构建二叉树 */
+    TreeNode? buildTree(List<int> preorder, List<int> inorder) {
+      // 初始化哈希表，存储 inorder 元素到索引的映射
+      Map<int, int> hmap = {};
+      for (int i = 0; i < inorder.length; i++) {
+        hmap[inorder[i]] = i;
+      }
+      TreeNode? root = dfs(preorder, inorder, hmap, 0, 0, inorder.length - 1);
+      return root;
+    }
     ```
 
 === "Rust"