leetcode124 -- Binary Tree Maximum Path Sum (Hard)

# 小郑之家~

### 题目


Given a non-empty 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.

Example 1:

Input: [1,2,3]

1
/ \
2   3

Output: 6
Example 2:

Input: [-10,9,20,null,null,15,7]

-10
/ \
9  20
/  \
15   7

Output: 42



### 解答


/**
* 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) {
int res = INT_MIN;
helper(root, res);
return res;
}

int helper(TreeNode* node, int& res){
if (!node) return 0;
int left = max(helper(node->left, res),0);
int right = max(helper(node->right, res),0);
res = max(res, left+right+node->val);
return max(left, right)+node->val;
}
};



        1
/       \
2               3



max(0, helper(node>left, res)), 右子树的贡献就是max(0, helper(node->right, res))