145 Binary Tree Postorder Traversal

Binary Tree Postorder Traversal

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

 

Example 1:

**Input:** root = [1,null,2,3]
**Output:** [3,2,1]

Example 2:

**Input:** root = []
**Output:** []

Example 3:

**Input:** root = [1]
**Output:** [1]

 

Constraints:

The number of the nodes in the tree is in the range [0, 100].
-100 <= Node.val <= 100

 

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

# 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 __init__(self):
        self.cache = []
    def postorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        if root == None:
            return
        
        self.postorderTraversal(root.left)
        self.postorderTraversal(root.right)
        self.cache.append(root.val)
        return self.cache