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/**
 * Definition for a binary tree node.
 * 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 maxSum = INT_MIN;
    int maxPathSum(TreeNode* root) {
        dfs(root);
        return maxSum;
    }

    int dfs(TreeNode *root){
        if(!root) return 0;
        int leftSum = 0, rightSum = 0, curSum = root->val;

        // Get the left subtree sum or 0 if negative.
        if(root->left)
            leftSum = max(leftSum, dfs(root->left));
        // Get the right subtree sum or 0 if negative.
        if(root->right)
            rightSum = max(rightSum, dfs(root->right));

        // Update answer.
        maxSum = max(maxSum, curSum+leftSum+rightSum);
        return max(leftSum+curSum, rightSum+curSum);
    }
};