LeetCode----Binary Tree Preorder Traversal

Binary Tree Preorder Traversal

Given a binary tree, return the preorder traversal of its nodes' values.

For example:
Given binary tree {1,#,2,3},

   1
    \
     2
    /
   3

return [1,2,3].

Note: Recursive solution is trivial, could you do it iteratively?

分析：

非递归先序遍历二叉树。用栈。

代码：

# 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]
        """
        st = []
        res = []
        p = root
        while p:
            res.append(p.val)
            if p.right:
                st.append(p.right)
            if p.left:
                st.append(p.left)
            if st:
                p = st.pop()
            else:
                break
        return res