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