545. Boundary of Binary Tree (Medium) - TengnanYao/daily_leetcode GitHub Wiki

# 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 boundaryOfBinaryTree(self, root: Optional[TreeNode]) -> List[int]:
        
        # root
        result = [root.val]
        
        # left boundary
        cur = root.left
        while cur:
            if cur.left or cur.right:
                result.append(cur.val)
            cur = cur.left or cur.right
            
        # leaves
        def dfs(node):
            if node:
                dfs(node.left)
                if node != root and not node.left and not node.right:
                    result.append(node.val)
                dfs(node.right)
        dfs(root)
        
        # right boundary
        cur = root.right
        temp = []
        while cur:
            if cur.left or cur.right:
                temp = [cur.val] + temp
            cur = cur.right or cur.left
        return result + temp