leetcode199. 二叉樹的右視圖

給定一棵二叉樹，想象自己站在它的右側，按照從頂部到底部的順序，返回從右側所能看到的節點值。

示例:
輸入: [1,2,3,null,5,null,4]
輸出: [1, 3, 4]

層序遍歷：

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

class Solution:
    def rightSideView(self, root: TreeNode) -> List[int]:
        if not root:
            return []
        res, stack = [], [root]
        while stack:  # 層序遍歷
            len_stack = len(stack)
            for i in range(len_stack):
                node = stack.pop(0)
                if i == len_stack-1:
                    res.append(node.val)
                if node.left:
                    stack.append(node.left)
                if node.right:
                    stack.append(node.right)
        return res

遞歸：

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

class Solution:
    def rightSideView(self, root: TreeNode) -> List[int]:
        if not root:
            return []
        self.res = [root.val]
        self.helper(root.right, 1)  # 先右後左
        self.helper(root.left, 1)
        return self.res
    
    def helper(self, node, height):
        if not node:
            return 
        if len(self.res)==height:
            self.res.append(node.val)
        self.helper(node.right, height+1)
        self.helper(node.left, height+1)