leetcode145

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]

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

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

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def postorderTraversal(self, root: TreeNode) -> List[int]:
        
        res = []
        stack = []
        p = root
        while len(stack)>0 or p:
            if p:
                stack.append(p)
                res.insert(0, p.val)
                p = p.right
            else:
                t = stack.pop()
                p = t.left
                
        return res