[Leetcode][python]Binary Tree Preorder Traversal/二叉树的前序遍历

            　　1

　　　　　　　/     \

　　　　　 2         3

　　　 /     \    /    \

　　  4       5  6      7 

class Solution(object):
    def _preorderTraversal(self, root, result):
        if root:
            result.append(root.val)
            self._preorderTraversal(root.left, result)
            self._preorderTraversal(root.right, result)
    def preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        if root == None:
            return []
        result = []
        self._preorderTraversal(root, result)
        return result

while stack:
    node = stack.pop()
    if node:
        ret.append(node.val)
        stack.append(node.right)
        stack.append(node.left)

<precompiled.treenode.TreeNode object at 0x7f19ec21fc90>
None
None

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