leetcode144 二叉树的前序遍历

给定一个二叉树，返回它的 前序 遍历。
示例:
输入: [1,null,2,3]
1

2
/
3

输出: [1,2,3]
方法1：
递归

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def _preorderTraversal(self,res,root):
        if not root:
            return
        res.append(root.val)
        self._preorderTraversal(res,root.left)
        self._preorderTraversal(res,root.right)
    
    def preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        res=[]
        self._preorderTraversal(res,root)
        return res

方法二：
堆栈

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        if not root:
            return []
        myStack = []
        res = []
        node = root
        while node or myStack:
            while node:                 #从根节点开始，一直找它的左子树
                myStack.append(node)
                res.append(node.val)
                node = node.left
            node = myStack.pop()        #while结束表示当前结点node为空，即前一个结点没有左子树了
            node = node.right 
        return res