content: /**
   * File: build_tree.dart
   * Created Time: 2023-08-10
   * Author: liuyuxin (gvenusleo@gmail.com)
   */
  
  import '../utils/print_util.dart';
  import '../utils/tree_node.dart';
  
  /* 構建二元樹：分治 */
  TreeNode? dfs(
    List<int> preorder,
    Map<int, int> inorderMap,
    int i,
    int l,
    int r,
  ) {
    // 子樹區間為空時終止
    if (r - l < 0) {
      return null;
    }
    // 初始化根節點
    TreeNode? root = TreeNode(preorder[i]);
    // 查詢 m ，從而劃分左右子樹
    int m = inorderMap[preorder[i]]!;
    // 子問題：構建左子樹
    root.left = dfs(preorder, inorderMap, i + 1, l, m - 1);
    // 子問題：構建右子樹
    root.right = dfs(preorder, inorderMap, i + 1 + m - l, m + 1, r);
    // 返回根節點
    return root;
  }
  
  /* 構建二元樹 */
  TreeNode? buildTree(List<int> preorder, List<int> inorder) {
    // 初始化雜湊表，儲存 inorder 元素到索引的對映
    Map<int, int> inorderMap = {};
    for (int i = 0; i < inorder.length; i++) {
      inorderMap[inorder[i]] = i;
    }
    TreeNode? root = dfs(preorder, inorderMap, 0, 0, inorder.length - 1);
    return root;
  }
  
  /* Driver Code */
  void main() {
    List<int> preorder = [3, 9, 2, 1, 7];
    List<int> inorder = [9, 3, 1, 2, 7];
    print("前序走訪 = $preorder");
    print("中序走訪 = $inorder");
  
    TreeNode? root = buildTree(preorder, inorder);
    print("構建的二元樹為：");
    printTree(root!);
  }
  