leetcode/Python/145_Binary_Tree_Postorder_Traversal.py at main · CPWu/leetcode · GitHub

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# 4 Possible Solutions
# 2. Iterative - Two Stack
# 3. Recursive

# 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:
    # Two Stack
    # Time: O(N), Space: O(H)
    def postorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        # Empty Tree
        if not root:
            return None

        stack = [root]
        result = []

        while stack:
            currentNode = stack.pop()
            result.append(currentNode.val)

            if currentNode.left:
                stack.append(currentNode.left)

            if currentNode.right:
                stack.append(currentNode.right)


        return result[::-1]

    # Recursion
    # Time: O(N), Space: O(H)
    def postorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        # Empty Tree
        if not root:
            return None

        result = []

        def _postOrderTraversal(node, result):
            if node:
                _postOrderTraversal(node.left, result)
                _postOrderTraversal(node.right, result)
                result.append(node.val)

        _postOrderTraversal(root, result)

        return result