01_二叉树的前序遍历

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None


class Solution(object):

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

    def preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        if root:
            self.ret.append(root.val)
            self.preorderTraversal(root.left)
            self.preorderTraversal(root.right)
        return self.ret


class Solution2(object):

    def preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        if not root:
            return []

        left = self.preorderTraversal(root.left)
        right = self.preorderTraversal(root.right)
        return [root.val] + left + right