Python实现二叉树的非递归先序遍历

# 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 preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        ret = []
        stack = []
        while root or stack:
            while root:
                ret.append(root.val)
                stack.append(root)
                root = root.left
            if stack:
                t = stack.pop()
                root = t.right
        return ret