543. Diameter of Binary Tree

# 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:
    longest = 0
    
    def diameterOfBinaryTree(self, root: Optional[TreeNode]) -> int:
        def dfs(node):
            if not node:
                return 0
            
            left = dfs(node.left)
            right = dfs(node.right)
            
            self.longest = max(self.longest, left + right)
            # 밑에서 올라갈때마다 + 1 씩 해줌
            return max(left, right) + 1
        
        dfs(root)
        return self.longest