计算BST中小于X的节点数

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

def countNodes(root, X):
    if not root:
        return 0
    
    count = 0
    stack = []
    current = root
    
    while current or stack:
        while current:
            stack.append(current)
            current = current.left
        
        current = stack.pop()
        
        if current.val < X:
            count += 1
        else:
            break
        
        current = current.right
    
    return count