Leetcode 144. Binary Tree Preorder Traversal

参考例子：[8,3,1,6,4,7,10,14,13]

8,3,1 和 6,4 说明从root开始，沿着左臂向下寻找leaf 的过程中应该逐个将node.val push入ans.

 class Solution(object):
     def preorderTraversal(self, root):
         """
         :type root: TreeNode
         :rtype: List[int]
         """
         stack = []
         ans = []
         while stack or root:
             while root:
                 stack.append(root)
                 ans.append(root.val)
                 root = root.left
             root = stack.pop()
             root = root.right
         return ans