動機

除了到根的，還有左右兩邊接起來的

Problem

Given the root of a binary tree, return the length of the diameter of the tree.

The diameter of a binary tree is the length of the longest path between any two nodes in a tree. This path may or may not pass through the root.

The length of a path between two nodes is represented by the number of edges between them.

 

Example 1:

Input: root = [1,2,3,4,5]Output: 3Explanation: 3 is the length of the path [4,2,1,3] or [5,2,1,3].

Example 2:

Input: root = [1,2]Output: 1

 

Constraints:

  • The number of nodes in the tree is in the range [1, 104].
  • -100 <= Node.val <= 100

Sol

用dfs求leaf到root的最長距離，同時在node更新最大值

class Solution:
    def dfs(self,r):
        if not r:
            return 0
        else:
            left = self.dfs(r.left)
            right = self.dfs(r.right)
            self.ret = max(self.ret, left+right)
            return max(left,right)+1
    def diameterOfBinaryTree(self, root: TreeNode) -> int:
        self.ret = 0
        self.dfs(root)
        return self.ret