Given a binary tree, find the maximum path sum.
The path may start and end at any node in the tree.
求树的一点到另一点的最大路径，利用递归的方法，ans 在 左子树，右子树，root+左+右的最大中产生。
/**
* 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 ans;
int maxPathSum(TreeNode *root) {
ans = INT_MIN;
travel(root);
return ans;
}
int travel(TreeNode * root){
if(root == NULL) return 0;
int sum = root->val;
int leftsum = travel(root->left);
int rightsum = travel(root->right);
if(leftsum > 0) sum += leftsum;
if(rightsum > 0) sum += rightsum;
ans = max(ans,sum);
sum = max(leftsum,rightsum);
return root->val + (sum > 0 ? sum : 0);
}
};