二分查找树遍历法，按序toString

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

public class BSTTraversal {
    public static String inorderToString(TreeNode root) {
        StringBuilder sb = new StringBuilder();
        inorderTraversal(root, sb);
        return sb.toString();
    }
    
    private static void inorderTraversal(TreeNode node, StringBuilder sb) {
        if (node == null) {
            return;
        }
        
        inorderTraversal(node.left, sb);
        sb.append(node.val).append(" ");
        inorderTraversal(node.right, sb);
    }
    
    public static void main(String[] args) {
        // 构建一个二分查找树
        TreeNode root = new TreeNode(4);
        root.left = new TreeNode(2);
        root.right = new TreeNode(6);
        root.left.left = new TreeNode(1);
        root.left.right = new TreeNode(3);
        root.right.left = new TreeNode(5);
        root.right.right = new TreeNode(7);
        
        // 按序转化为字符串并输出
        String result = inorderToString(root);
        System.out.println(result);  // 输出：1 2 3 4 5 6 7
    }
}