leetcode-二叉树的前序遍历

144. 二叉树的前序遍历

递归方法

# 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: Optional[TreeNode]) -> List[int]:
        res = []
        def helper(root, res):
            if root:
                res.append(root.val)
                helper(root.left, res)
                helper(root.right, res)
        helper(root, res)
        return res

迭代法

# 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: Optional[TreeNode]) -> List[int]:
        if not root:
            return []
        stack, res = [root], []
        while stack:
            node = stack.pop()
            res.append(node.val)
            # 右孩子先入栈，左孩子后入栈，这样出栈的时候就是中左右
            if node.right:
                stack.append(node.right)
            if node.left:
                stack.append(node.left)
        return res