144. Binary Tree Preorder Traver

话不多说，直接上栈的解法。
直接打印根节点的值，然后将根节点入栈，然后将根节点的左节点循环入栈。
然后弹出栈顶元素，再去遍历其右节点。

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def preorderTraversal(self, root: TreeNode) -> List[int]:
        result = []
        if not root:
            return result
        p = root
        stack = []
        while len(stack) > 0 or p:
            if p:
                result.append(p.val)
                stack.append(p)
                p = p.left
            else:
                p = stack.pop()
                p = p.right
        return result
                
                
                
            
        

为了练手，还是写一下递归的写法

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def preorderTraversal(self, root: TreeNode) -> List[int]:
        result = []
        if not root:
            return result
        result.append(root.val)
        result += self.preorderTraversal(root.left)
        result += self.preorderTraversal(root.right)
        return result