543.cpp

#include <bits/stdc++.h>
using namespace std;

struct TreeNode {
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode() : val(0), left(nullptr), right(nullptr) {}
    TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
    TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
};

class Solution {
public:
    int diameterOfBinaryTree(TreeNode* root) {
        int res = 0;
        dfs(root, res);
        return res;
    }

private:
    int dfs(TreeNode* root, int& res) {
        if (!root) return 0;

        int left = dfs(root->left, res);
        int right = dfs(root->right, res);

        res = max(res, left+right);

        return 1 + max(left, right);
    }
};