LeetCode 515: Find Largest Value in Each Tree Row

Meta Data

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Difficulty: medium First Attempt: 2026-10-24

• Submissions: Accepted (first try)

Approach

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BFS level order (Tree Template 2), keeping one maximum per level. Values can be negative (down to -2^31), so each level starts from -inf.

My Attempts

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Accepted on the first try.

Solution

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class Solution:
    def largestValues(self, root: Optional[TreeNode]) -> List[int]:
        if not root:
            return []
        res = []
        queue = deque([root])
        while queue:
            best = -inf
            for _ in range(len(queue)):
                node = queue.popleft()
                best = max(best, node.val)
                if node.left:
                    queue.append(node.left)
                if node.right:
                    queue.append(node.right)
            res.append(best)
        return res

Complexity

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Time O(n), space O(w).

Mistakes to Avoid

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• Starting each level’s maximum at 0 is wrong when a whole level is negative.
• Return [] for an empty tree.

Interview Explanation

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I go level by level with BFS and take the maximum of each level starting from -inf, since values can be negative. Time O(n), space is the widest level. A DFS with depth works too: res[depth] = max(res[depth], val).