515. Find Largest Value in Each Tree Row

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You need to find the largest value in each row of a binary tree.
Example:
Input: 

          1
         / \
        3   2
       / \   \  
      5   3   9 

Output: [1, 3, 9]
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Time - O(n)
Space - O(2 ^ h) - # nodes at last level
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