非递归先序遍历二叉树, 弹栈print, 右子节点入栈, 再左子节点入栈。

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

class Solution:
    # @param root, a tree node
    # @return a list of integers
    def preorderTraversal(self, root):
        res = []
        if root == None:
            return res
        stack = [root]
        while stack:
            root = stack.pop()
            res.append(root.val)
            if root.right:
                stack.append(root.right)
            if root.left:
                stack.append(root.left)
        return res