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树形动态规划 Tree DP

543. Diameter of Binary Tree​

dfs(node) = max(dfs(node), dfs(node.left) + dfs(node.right) + 2)

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def diameterOfBinaryTree(self, root: Optional[TreeNode]) -> int:
        ans = 0
        def calc(node):
            if node is None:
                return -1
            rLen = calc(node.right)
            lLen = calc(node.left)
            nonlocal ans
            ans = max(ans, lLen + rLen + 2)
            return max(rLen, lLen) + 1
        calc(root)
        return ans

Reference​

  1. 树形 DP：树的直径【基础算法精讲 23】