144.py

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

	Example:

	Input: [1,null,2,3]
	   1
	    \
	     2
	    /
	   3

	Output: [1,2,3]
	Follow up: Recursive solution is trivial, could you do it iteratively?
'''

# 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]
        """
        if not root:
        	return []

        stack, result = [root], []
        while stack:
        	element = stack.pop()
        	result.append(element.val)

        	if element.right:
        		stack.append(element.right)
        	if element.left:
        		stack.append(element.left)

        return result


# 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]
        """
        result = []

        def recursive(root, result):
        	if not root:
        		return 
        	result.append(root.val)
        	recursive(root.left, result)
        	recursive(root.right, result)

        recursive(root, result)
        return result