199. Binary Tree Right Side View

You can do this with either BFS or DFS. BFS goes level by level so it’s natural for this:

We can then do the level by level traversal while replacing the last node we see in each level and return the last number for each level.

class Solution:
    def rightSideView(self, root: Optional[TreeNode]) -> List[int]:
        q = deque()
        q.append(root)
        res = defaultdict(int)
        l = -1 
 
        while q:
            l += 1
            for _ in range(len(q)):
                node = q.popleft()
                if not node:
                    continue
                res[l] = node.val
                q.append(node.left)
                q.append(node.right)
        
        return [res[i] for i in range(l)]