543. Diameter of Binary Tree.py

# 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:
        def diameter(node, res):
            if not node:
                return 0
            left = diameter(node.left, res)
            right = diameter(node.right, res)

            res[0] = max(res[0], left + right)
            
            return max(left, right) + 1
        
        res = [0]
        diameter(root, res)
        return res[0]