Binary Tree Maximum Path Sum

Given a binary tree, find the maximum path sum.

The path may start and end at any node in the tree.

For example:
Given the below binary tree,

       1
      / 
     2   3

Return 6.

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int maxPathSum(TreeNode *root) 
    {
        if(root==NULL) return 0;
        int allmax=root->val;
        getmax(root,allmax);
        return allmax;
    }
    int getmax(TreeNode *root,int& allmax) 
    {
        if(root==NULL) return 0;
        
        int line=root->val;
        int left=getmax(root->left,allmax);
        int right=getmax(root->right,allmax);
        if(left>right && left>=0) line+=left;
        if(left<right && right>=0) line+=right;
        
        int sum=root->val;
        if(left>=0) sum+=left;
        if(right>=0) sum+=right;
        if(allmax<sum) allmax=sum;
        
        return line;
    }
};