binary-tree-maximum-path-sum

#include<iostream>
#include<cmath>
#include<algorithm>
 struct TreeNode {
     int val;
     TreeNode *left;
     TreeNode *right;
     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 };

class Solution {
public:
    int maxPathSum(TreeNode *root) {
        maxx(root);
        return maxn;
    }
    int maxx(TreeNode *root)
    {
        if (root == NULL)return 0;
        int num1 = max(0, maxx(root->left));        //注意将为负数的子树拆掉
        int num2 = max(0, maxx(root->right));
        maxn = max(maxn, num1 + num2 + root->val);
        return max(num1, num2) + root->val;            //注意虽然为负数，但是也要返回，因为把它当成一颗数来看
    }
    int max(int a, int b){
        return a > b ? a : b;
    }
    int maxn=-0x3f3f3f3f;
};