545. Boundary of Binary Tree

# 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:
    # time O(N) space O(N)
    def boundaryOfBinaryTree(self, root: TreeNode) -> List[int]:
        # order root=> left boundry ==> leaf nodes (not include leaf nodes) ==> right boundry(not include leaf nodes) in reverse order  
        res = []
        # step1 define check leaf node func
        def isLeaf(node):
            if not node.left and not node.right:
                return True
            return False 
        
        # step2 define add leaves recursive function 
        def addLeaves(res,node):
            if isLeaf(node):
                res.append(node.val)
            else:
                if node.left:
                    addLeaves(res,node.left)
                if node.right:
                    addLeaves(res,node.right)
                
        if not root:
            return res
        # step2: add root 
        if not isLeaf(root):
            res.append(root.val)
        # step3: get left boundry
        t = root.left
        while t:
            # add non-leaf node's value into <<res>>
            if not isLeaf(t):
                res.append(t.val)
                print(res,'add left boudry no leaf nodes ')
            if t.left:
                t = t.left
            else:
                t = t.right
        
        #step4 add leaves
        addLeaves(res,root)
        print(res,'add leaves')
        
        #step5: get right boundry
        st = []
        t = root.right
        while t:
            # push non- leaf node'value into <<stack>> 
            if not isLeaf(t):
                st.append(t.val)
            if t.right:
                t = t.right
            else:
                t = t.left 
        while st:
            res.append(st.pop())
            print(res,'right boudry  no leaf nodes reverse order')
        return res