举例：如下图，124+125+136 = 386

int sumTree(TreeNode* node,int sum)
    {
        if(node==nullptr)
        {
            return 0;
        }
        if(node->left==nullptr && node->right==nullptr)
        {
            sum*=10;
            sum+=node->val;
            return sum;
        }
        sum *=10;
        sum += node->val;
        int sum_left = sumTree(node->left,sum);
        int sum_right = sumTree(node->right,sum);
        return sum_left+sum_right;
    }
    int sum(TreeNode* root) {
        return sumTree(root,0);
    }