124. Binary Tree Maximum Path Sum (Tree; DFS)

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int maxPathSum(TreeNode *root) {
        maxSum = INT_MIN;
        calRootSum(root);
        
        return maxSum;
        
    }
    
    int calRootSum(TreeNode *root)
    {
        int leftSum = 0;
        int rightSum = 0;
        int sum;
        
        if(root->left) 
        {
            leftSum = calRootSum(root->left); //traverse left subtree
        }
        if(root->right) 
        {
            rightSum = calRootSum(root->right);//traverse right subtree
        }
        
        //val小于零的情况
        if(leftSum < 0) 
        {
            leftSum = 0;
        }
        if(rightSum < 0)
        {
            rightSum = 0;
        }
        
        sum = root->val+leftSum+rightSum;
        if(sum > maxSum)
        {
            maxSum = sum;
        }
        
        sum = root->val+max(leftSum,rightSum);
        return sum;
    }
private:
    int maxSum;
};