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LC515 - Find Largest Value in Each TreeRow

Posted on In
Leetcode: 140/2658 (57+67+16)

Description

Given the root of a binary tree, return an array of the largest value in each row of the tree (0-indexed).
Example 1:

Input: root = [1,3,2,5,3,null,9]
Output: [1,3,9]

Example 2:
Input: root = [1,2,3]
Output: [1,3]

Constraints:
The number of nodes in the tree will be in the range [0, 104].
-231 <= Node.val <= 231 - 1

Solution

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# O(n) time | O(n) space
# 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 largestValues(self, root: Optional[TreeNode]) -> List[int]:
        if root is None:
            return []
        
        queue = collections.deque([root])
        ans = []
        while queue:
            l = len(queue)
            maxi = float('-inf')
            for i in range(l):
                pop_node = queue.popleft()
                maxi = max(maxi, pop_node.val)
                if pop_node.left:
                    queue.append(pop_node.left)
                if pop_node.right:
                    queue.append(pop_node.right)
            ans.append(maxi)
        return ans