LeetCode【binary-tree-maximum-path-sum】

思路：
不斷地返回左右子樹的兩邊的最大值，分清最大值和遞歸函數的返回值。

/**
 * 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 max_sum;
    int maxPathSum(TreeNode *root) {
        max_sum = INT_MIN;
        dfs(root);
        return max_sum;
    }
    int dfs(TreeNode *root)
    {
        if (root == NULL)
            return 0;
        int sum = root->val;
        int leftVal = dfs(root->left);
        int rightVal = dfs(root->right);
        if (leftVal > 0)
            sum += leftVal;
        if (rightVal > 0)
            sum += rightVal;
        max_sum = max(max_sum, sum);            //最大值
        return max(r,l)>0 ? max(r, l)+root->val : root->data;
    }
};