LeetCode #515 Find Largest Value in Each Tree Row

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Problem

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

Example

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Input: 

          1
         / \
        3   2
       / \   \  
      5   3   9 

Output: [1, 3, 9]

Solution1

Method: Breadth-first Search

Time Complexity: O(n)

Space Complexity:

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# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def largestValues(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        if not root:
            return []
        
        ret = []
        dq = collections.deque([(root, 0)])
        row_maximum = [float('-inf'), 0]
        
        while dq:
            node, level = dq.popleft()
            if not node:
                continue
            if level == row_maximum[1]:
                row_maximum[0] = max(node.val, row_maximum[0])
            else:
                ret.append(row_maximum[0])
                row_maximum = [node.val, level]
            dq.append((node.left, level + 1))
            dq.append((node.right, level + 1))

        ret.append(row_maximum[0])
        return ret

Solution2

Method: Depth-first Search

Time Complexity: O(n)

Space Complexity:

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class Solution:
    def largestValues(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        ret = []
        bag = [(root, 1)]
        
        while bag:
            node, level = bag.pop()
            if not node:
                continue

            if level > len(ret):
                ret.append(node.val)
            else:
                if ret[level - 1] < node.val:
                    ret[level - 1] = node.val

            bag.append((node.right, level + 1))
            bag.append((node.left, level + 1))

        return ret