LeetCode 124. Binary Tree Maximum Path Sum

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题目

Given a non-empty binary tree, find the maximum path sum.

For this problem, a path is defined as any sequence of nodes from some starting node to any node in the tree along the parent-child connections. The path must contain at least one node and does not need to go through the root.

Example 1:

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Input: [1,2,3]

       1
      / \
     2   3

Output: 6

Example 2:

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Input: [-10,9,20,null,null,15,7]

   -10
   / \
  9  20
    /  \
   15   7

Output: 42

思路

DFS.
注意节点值为负的时候跳过(取0)。

代码

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# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution(object):
    def maxPathSum(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        self.ans = float('-inf')
        self.dfs(root)
        return self.ans
    
    def dfs(self, root):
        if root is None:
            return 0
        left = self.dfs(root.left)
        right = self.dfs(root.right)
        self.ans = max(self.ans, root.val+max(left, 0)+max(right, 0))
        return max(left, right, 0)+root.val