【剑指offer】二叉搜索树的第k个结点

public class  {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;
    
    public (int val) {
        this.val = val;
    }
}

private int count = 0;

public TreeNode KthNode(TreeNode pRoot, int k) {
    if (pRoot == null || k < 1)
        return null;

    return KthNodeHelper(pRoot, k);
}

private TreeNode KthNodeHelper(TreeNode root, int k) {
    if (root == null)
        return null;

    TreeNode node = KthNodeHelper(root.left, k);

    if (node != null)
        return node;

    if (++count == k)
        return root;

    node = KthNodeHelper(root.right, k);

    if (node != null)
        return node;

    return null;
}