[LeetCode] 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.

参考资料：

　　1. http://www.cnblogs.com/shawnhue/archive/2013/06/08/leetcode_124.html

　　2. http://www.cnblogs.com/x1957/p/3275854.html

/**
 * 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 scanT(TreeNode* root){
        if(root == NULL) return 0;
        int left = scanT(root -> left);
        int right = scanT(root -> right);
        int val = root -> val;
        if(left > 0) val += left;
        if(right > 0) val += right;
        if(val > ans) ans = val;
        return max(root->val ,max(left +  root -> val , right + root -> val));
    }
    int maxPathSum(TreeNode *root) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if(root == NULL) return 0;
        ans = root -> val;
        scanT(root);
        return ans;
    }
};