二叉树的后序遍历（非递归方法）

# 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 postorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        if root ==None:
            return []
        stack=[root]
        pre=None
        result=[]
        while (len(stack)!=0):
            cur =stack[-1]
            if ((cur.left ==None and cur.right==None) or (pre!=None and (pre==cur.left or  pre== cur.right))):
                
                cur=stack.pop()
                result.append(cur.val)
                pre=cur
            else:
                if cur.right!=None:
                    stack.append(cur.right)
                if cur.left!=None:
                    stack.append(cur.left)
        return result