原题

已知一颗二叉树，找到从根节点到任意节点的最大路径和

已知下面一颗二叉树

  1
 / \
2   3

返回4. (1->3)

解题思路

• Divide and Conquer
• 借助一个helper函数，从根节点向下分裂，每一层返回左右儿子中的最大值与当前节点的值，层层递归

完整代码

"""
Definition of TreeNode:
class TreeNode:
    def __init__(self, val):
        this.val = val
        this.left, this.right = None, None
"""
class Solution:
    """
    @param root the root of binary tree.
    @return an integer
    """
    def maxPathSum2(self, root):
        return self.helper(root)
        
    def helper(self, root):
        if not root:
            return 0
            
        left = self.helper(root.left)
        right = self.helper(root.right)
        
        return max(left, right) + root.val