Binary Tree Maximum Path Sum

Given a binary tree, find the maximum path sum.

For this problem, a path is defined as any sequence of nodes from some starting node to any node in the tree along the parent-child connections. The path must contain at least one node and does not need to go through the root.

For example:
Given the below binary tree,

   1  / \ 2   3

 class Solution {public:    int maxPathSum(TreeNode* root) {        if(!root) return 0;       int m=-2147483648;       maxPath(root,m);      return m;    }    int maxPath(TreeNode* root,int &m){//这里的m要是引用        if(!root) return 0;         int a=maxPath (root->left,m);        int b=maxPath(root->right,m);        if(a<0) a=0;        if(b<0) b=0;        if(a+b+root->val>m) m=a+b+root->val;        return root->val+max(a,b);    }};