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

def dfs(root):
    if root is None:
        return
    s = []
    s.append(root)
    while len(s) > 0:
        node = s.pop()
        print(node.value, end=" ")
        # --------此处填空--------