11.10

二叉树：

1. 节点类定义

java
class TreeNode {
int val;
TreeNode left; // 左子节点
TreeNode right; // 右子节点

public TreeNode(int val) {
    this.val = val;
    this.left = null;
    this.right = null;
}

}

2. 构建与遍历
   java
   public class BinaryTree {
   // 前序遍历：根→左→右
   public static void preOrder(TreeNode root) {
   if (root == null) return;
   System.out.print(root.val + " ");
   preOrder(root.left);
   preOrder(root.right);
   }

   public static void main(String[] args) {
   // 构建二叉树
   TreeNode root = new TreeNode(1);
   root.left = new TreeNode(2);
   root.right = new TreeNode(3);
   root.left.left = new TreeNode(4);
   root.left.right = new TreeNode(5);

    // 遍历测试
    System.out.print("前序遍历：");
    preOrder(root); // 输出：1 2 4 5 3
   

   }
   }