LeetCode 筆記 - 199. Binary Tree Right Side View

# 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 rightSideView(self, root: Optional[TreeNode]) -> List[int]:
        
        if not root:
            return []
        
        
        upper = [root]
        lower = []
        
        result = []
        while upper:

            result.append(upper[-1].val)
            while upper:
                current_node = upper.pop(0)

                if current_node.left:
                    lower.append(current_node.left)
                if current_node.right:
                    lower.append(current_node.right)
            
            upper = lower
            lower = []
            
        return result