199. Binary Tree Right Side View刷题笔记

运行速度慢主要是pop(0)耗费大量时间，而deque没办法存放TreeNode类型节点

# 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 []
        que = [root]
        res = []
        while True:
            length = len(que)
            if not length:
                return res
            res.append(que[0].val)                
                
            for _ in range(length):
                root = que.pop(0)
                if root.right:
                    que.append(root.right)
                if root.left:
                    que.append(root.left)
        return res