二叉树的前序遍历

    def __init__(self):
        self.res = []

    def PreOrder(self, root):
        self.dfs(root)
        return self.res

    def dfs(self, root):
        if not root:
            return
        self.res.append(root.val)
        self.dfs(root.left)
        self.dfs(root.right)

    def PreOrder2(self, root):
        # 非递归前序遍历
        stack = []
        cur = root
        while stack or cur:
            while cur:
                self.res.append(cur.val)
                stack.append(cur)
                cur = cur.left
            cur = stack.pop()
            cur = cur.right
        return self.res