Printing Level Order Binary Search Tree Formatting

Question

I have implemented the following code to print a binary search tree in level order.

public void printLevelOrder(int depth) {
    for (int i = 1; i <= depth; i++) {
        printLevel(root, i);
    }
}

public void printLevel(BinaryNode<AnyType> t, int level) {
    if (t == null) {
        return;
    }
    if (level == 1) {
        System.out.print(t.element);
    } else if (level > 1) {
        printLevel(t.left, level - 1);
        printLevel(t.right, level - 1);
    }
}

I am trying to figure out how to improve my code to have it print out in a certain format.

As an example, given a tree

    1 
   / \
  2   3
 /   / \
4   5   6

Currently it prints like so:

123456

I am looking for it to print as follows:

Level 0: 1
Level 1: 2 3
Level 2: 4 5 6

Answer 1

Instead of printing the values immediately inside the recursive function calls, use strings to hold the values. This will make it easier to manipulate the output.

public void printLevelOrder(int depth) {
    for (int i = 1; i <= depth; i++) {
        System.out.print("Level " + (i-1) + ": ");
        String levelNodes = printLevel(root, i);
        System.out.print(levelNodes + "\n");
    }
}

public String printLevel(BinaryNode<AnyType> t, int level) {
    if (t == null) {
        return "";
    }
    if (level == 1) {
        return t.element + " ";
    } else if (level > 1) {
        String leftStr = printLevel(t.left, level - 1);
        String rightStr = printLevel(t.right, level - 1);
        return leftStr + rightStr;
    }
    else // you need this to get it to compile
      return "";
}

Output:

Level 0: 1 
Level 1: 2 3 
Level 2: 4 5 6