Binary Tree Maximum Path Sum

https://leetcode.com/problems/binary-tree-maximum-path-sum

# Definition for a Binary Tree Node

Definition of TreeNode:
class TreeNode:
    def __init__(self, val):
        self.val = val
        self.left, self.right = None, None

# Solution

# Recursive DFS

Return 2 values in dfs:

  1. max_sum is the sum of tree of current node as highest node
  2. max_chain is sum of current node plus either the left or the right sub-tree

At the end, return the max_sum of root, as root must be included.

class Solution:
    """
    @param root: The root of binary tree.
    @return: An integer
    """
    def maxPathSum(self, root):
        def dfs(root):
            if not root: return (-sys.maxsize, 0)

            left_max_sum, left_max_chain = dfs(root.left)
            right_max_sum, right_max_chain = dfs(root.right)
            max_sum = max(left_max_sum, right_max_sum, left_max_chain + right_max_chain + root.val)
            max_chain = max(max(left_max_chain, right_max_chain) + root.val, 0)
            return (max_sum, max_chain)
        return dfs(root)[0]

# Improved Recursive DFS

This solution has similar logic but easier to understand. Update res if max_sum is larger. Return max_chain.

def maxPathSum(self, root: TreeNode) -> int:
        def dfs(node):
            nonlocal res
            if not node:
                return 0
            # max sum on left/right sub-trees of node
            left_sum = max(dfs(node.left), 0)
            right_sum = max(dfs(node.right), 0)

            max_sum = node.val + left_sum + right_sum
            res = max(res, max_sum)
            return node.val + max(left_sum, right_sum)
        res = -sys.maxsize
        dfs(root)
        return res
Last Updated: 9/21/2020, 4:44:16 PM