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

def dfs(root):
    if not root:
        return

    _________________________
    stack.append(root)

    while stack:
        node = stack.pop()
        print(node.val, end=" ")

        if node.right:
            stack.append(node.right)
        if node.left:
            stack.append(node.left)