class Solution {
    int maxLen = -1;
    int depth(TreeNode* root) {
        if (!root) return 0;
        int L = depth(root->left);
        int R = depth(root->right);
        maxLen = max(maxLen, L + R);
        return max(L, R) + 1;
    }
public:
    int diameterOfBinaryTree(TreeNode* root) {
        depth(root);
        return maxLen;
    }
};

[104. 二叉树的最大深度](https://qiekn.notion.site/104-02e8f766ebd144158a46e1ebe984d153)