https://leetcode.com/problems/diameter-of-binary-tree

diameter_of_binary_tree.cpp

class Solution {
private:
    struct Result { int diameter; int depth; };
    Result maxDiameterAndDepth(const TreeNode* root) const {
        if (root == nullptr) return {0, 0};
        auto leftResult = maxDiameterAndDepth(root->left);
        auto rightResult = maxDiameterAndDepth(root->right);
        return {
            max({leftResult.diameter, rightResult.diameter, leftResult.depth + rightResult.depth}),
            1 + max(leftResult.depth, rightResult.depth)
        };
    }
public:
    int diameterOfBinaryTree(const TreeNode* root) const {
        return maxDiameterAndDepth(root).diameter;
    }
};