Max Sum Path in Binary Tree

• Time : $O(n)$
• Space : $O(1)$

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
 int res;
 int dfs(TreeNode* A) {
     if(A == NULL) return 0;
     int l = dfs(A->left), r = dfs(A->right);
     res = max(res, l + r + A->val);
     res = max(res, max({l,r,0}) + A->val);
     return max({l,r,0}) + A->val;
 }
int Solution::maxPathSum(TreeNode* A) {
    res = INT_MIN;
    dfs(A);
    return res;
}