Binary Tree: Number of Nodes at a Particular Level

Question

Hi, I'm trying to teach myself about binary tree but am having trouble writing an algorithm. The algorithm that I'm trying to write is one that will take a binary tree and output the number of nodes at each level of the tree (maybe in an array).
I have no trouble writing an algorithm that does a breadth-first traversal, but I have lots of trouble trying to determine where each level ends.
Thanks for the help

Answer 1

Try something like this:
class BTree {
    BTreeNode root;
    public int numberOfNodesAtLevel(int level) {
        if(root == null) return -1;
        return numberOfNodesAtLevel(root, level);
    private int numberOfNodesAtLevel(BTreeNode node, int level) {
        if(level == 0) {
            return 1;
        } else {
            return (node.left  == null ? 0 : numberOfNodesAtLevel(node.left, level-1)) +
                   (node.right == null ? 0 : numberOfNodesAtLevel(node.right,level-1));
}