199-binary-tree-right-side-view

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Description

Given the root of a binary tree, imagine yourself standing on the right side of it, return the values of the nodes you can see ordered from top to bottom.

 

Example 1:

Input: root = [1,2,3,null,5,null,4]
Output: [1,3,4]

Example 2:

Input: root = [1,null,3]
Output: [1,3]

Example 3:

Input: root = []
Output: []

 

Constraints:

• The number of nodes in the tree is in the range [0, 100].
• -100 <= Node.val <= 100

Solution(Python)

# 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]:
        return self.dfs(root)
        
    # Time Complexity: O(n)
    # Space Complexity: O(n)
    def levelorder(self, root: Optional[TreeNode]) -> List[int]:
        res = []
        q = deque([root])
        while q:
            n = len(q)
            for i in range(n):
                node = q.popleft()
                if  i== n-1:
                    res.append(node.val)
                if node.left:
                    q.append(node.left)
                if node.right:
                    q.append(node.right)
        return res
    
    # Time Complexity: O(n)
    # Space Complexity: O(H)
    def dfs(self, root: Optional[TreeNode]) -> List[int]:
        res = []
        heights = {}
        def preorder(node, h):
            if node:
                if h not in heights:
                    heights[h] = True
                    res.append(node.val)
                preorder(node.right, h+1)
                preorder(node.left, h+1)
                
                
        preorder(root,0)
        return res