java Printing inorder BST with parenthesis

Question

I want to return a String containing all keys in the tree, in the order they are stored. The keys in each subtree should be contained in a parenthesis.

        _7_
      /     \
   _3_      8
 /     \
1       6
 \     /
  2   4
       \
        5

The output for this BST should be (((()1(()2()))3((()4(()5()))6()))7(()8())). My code for doing this is:

public String printKeysInOrder() {
    if (isEmpty()) {
        return "()";
    }

    printInOrderRec(root);

    System.out.print(sb.toString());
    return sb.toString();
}

StringBuilder sb = new StringBuilder();

private String printInOrderRec(Node root) {
    if (root == null) {
        return null;
    }
    sb.append("(");
    printInOrderRec(root.left);
    sb.append("(");
    sb.append(")");

    sb.append(root.val);

    printInOrderRec(root.right);

    return null;
}

Which gives me the output: (((()1(()2()3((()4(()5()6()7(()8. I have been working on this for ages and can't figure out where and how to append the missing brackets. Any help would be appreciated.

Answer 1

Before jumping into coding solution, let's try to draw how the output should be can be generated.

(--------------------------------7-------)
 (------------3-----------------) (--8--)
  (--1-------) (------------6--)   () ()
   () (--2--)   (--4-------) ()
       () ()     () (--5--)
                     () ()

Here every enclosed pair of parentheses defines a call stack. I am not trying to describe each call stack, otherwise this answer would be arbitarily long. However, from the drawing we can find out 5 portions at each call stack.

1. Left paranthesis
2. Left child
3. Value
4. Right child
5. Right paranthesis

So, the your printInOrderRec method may look like:

private void printInOrderRec(Node root) {
    sb.append("(");
    if (root != null) {
        printInOrderRec(root.left);
        sb.append(root.val);
        printInOrderRec(root.right);
    }
    sb.append(")");
}

Note: I have made the return type void because in your code it returns nothing but null.