A relatively difficult tree problem. Well, a recursive solution still gives clean codes. The tricky part of this problem is how to record the result. You may refer to for a nice solution. The code is rewritten as follows.

class Solution {public:    int maxPathSum(TreeNode* root) {        sum = INT_MIN;        pathSum(root);        return sum;    }private:    int sum;    int pathSum(TreeNode* node) {        if (!node) return 0;        int left = max(0, pathSum(node -> left));        int right = max(0, pathSum(node -> right));        sum = max(sum, left + right + node -> val);        return max(left, right) + node -> val;    }};