145. Binary Tree Postorder Traversal

Hard

Given a binary tree, return the postorder traversal of its nodes’ values.

Example:

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

Output: [3,2,1]

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

1. Recursive

class Solution:
    def postorderTraversal(self, root: TreeNode) -> List[int]:
        if not root: return []
        res = []
        self.dfs(root, res)
        return res
    
    def dfs(self, node, res):
        if node:
            self.dfs(node.left, res)
            self.dfs(node.right, res)
            res.append(node.val)