geeksforgeeks Find the maximum path sum between two leaves of a binary tree

Geeksforgeeks上面的一道题： http://www.geeksforgeeks.org/find-maximum-path-sum-two-leaves-binary-tree/
Geeksforgeeks题目描述和解答已经非常清楚，只是那里的实现有bug，见后面的评论。在所有节点val为负或者只有一条边其 val为负的情况下，原实现都返回错误结果。原因是为空的时候返回0这个简单设想覆盖了正确答案。尽管很多人指出了这个问题，但截止目前(2014/7/27)，geeksforgeeks还没有更正。

#include <iostream>
using namespace std;

// #define INT_MIN     (-2147483647 - 1)

struct TreeNode
{
int val;
TreeNode *left;
TreeNode *right;
TreeNode (int val)
{
this->val = val;
this->left = NULL;
this->right = NULL;
}
};

class Solution {
public:
int maxPathSum(TreeNode *root)
{
int maxsum = INT_MIN;
maxPathSum(root, maxsum);
return maxsum;
}

int maxPathSum(TreeNode *root, int &maxsum)
{
if (root->left == NULL && root->right == NULL)
{
maxsum = max(maxsum, root->val);
return root->val;
}
else if (root->left == NULL)
{
int r = maxPathSum(root->right, maxsum);
maxsum = max(maxsum, r);
return root->val + r;
}
else if (root->right == NULL)
{
int l = maxPathSum(root->left, maxsum);
maxsum = max(maxsum, l);
return l + root->val;
}
else
{
int l = maxPathSum(root->left, maxsum);
int r = maxPathSum(root->right, maxsum);
maxsum = max(maxsum, l + root->val + r);
return max(l, r) + root->val;
}
}

void printTree(TreeNode *root)
{
if (root)
{
cout<<root->val<<endl;
printTree(root->left);
printTree(root->right);
}
}
};

int main()
{
// TreeNode *root = new TreeNode(-15);
// root->left = new TreeNode(5);
// root->right = new TreeNode(6);

// root->left->left = new TreeNode(-8);
// root->left->right = new TreeNode(1);
// root->left->left->left = new TreeNode(2);
// root->left->left->right = new TreeNode(6);

// root->right->left = new TreeNode(3);
// root->right->right = new TreeNode(9);
// root->right->right->right = new TreeNode(0);
// root->right->right->right->left = new TreeNode(4);
// root->right->right->right->right = new TreeNode(-1);
// root->right->right->right->right->left = new TreeNode(10);

// TreeNode *root = new TreeNode(-3);
// printTree(root);

TreeNode *root = new TreeNode(-3);
root->right = new TreeNode(8);
root->left = new TreeNode(4);

Solution s;
cout<<s.maxPathSum(root)<<endl;

return 0;
}