Find largest value in each tree row

Time: O(N); Space: O(H); medium

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

Example 1:

Input: root = {TreeNode} [1,3,2,5,3,None,9]

    1
   / \
  3   2
 / \   \
5   3   9

Output: [1,3,9]

Example 2:

     1
   /    \
  2      3
 / \    /
4   5  6
 \
  7

Input: root = {TreeNode} [1,2,3,4,5,6,None,None,7]

Output: [1,3,6,7]

class TreeNode(object):
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

1. Recursion

class Solution1(object):
    """
    Time: O(N)
    Space: O(H)
    """
    def largestValues(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        def largestValuesHelper(root, depth, result):
            if not root:
                return

            if depth == len(result):
                result.append(root.val)
            else:
                result[depth] = max(result[depth], root.val)

            largestValuesHelper(root.left, depth + 1, result)
            largestValuesHelper(root.right, depth + 1, result)

        result = []
        largestValuesHelper(root, 0, result)

        return result

s = Solution1()

root = TreeNode(1)
root.left, root.right = TreeNode(3), TreeNode(2)
root.left.left, root.left.right = TreeNode(5), TreeNode(3)
root.right.right = TreeNode(9)
assert s.largestValues(root) == [1,3,9]

root = TreeNode(1)
root.left, root.right = TreeNode(2), TreeNode(3)
root.left.left, root.left.right = TreeNode(4), TreeNode(5)
root.right.left = TreeNode(6)
root.left.left.right = TreeNode(7)
assert s.largestValues(root) == [1,3,6,7]

2. Iterative

class Solution2(object):
    def largestValues(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        result = []
        curr = [root]
        while any(curr):
            result.append(max(node.val for node in curr))
            curr = [child for node in curr for child in (node.left, node.right) if child]
        return result

s = Solution2()

root = TreeNode(1)
root.left, root.right = TreeNode(3), TreeNode(2)
root.left.left, root.left.right = TreeNode(5), TreeNode(3)
root.right.right = TreeNode(9)
assert s.largestValues(root) == [1,3,9]

root = TreeNode(1)
root.left, root.right = TreeNode(2), TreeNode(3)
root.left.left, root.left.right = TreeNode(4), TreeNode(5)
root.right.left = TreeNode(6)
root.left.left.right = TreeNode(7)
assert s.largestValues(root) == [1,3,6,7]

See also:

https://leetcode.com/problems/find-largest-value-in-each-tree-row

https://www.lintcode.com/problem/find-largest-value-in-each-tree-row/description