void allPath(TreeNode*root, vector<string>&result, vector<int>&path)

    {

        if (root==NULL)

            return ;

        path.push_back(root->val);

        if (root->left==NULL && root->right==NULL)

        {

//注意这里要用ostringstream将字符串写到流中去，然后用str（）函数返回字符串

            ostringstream sstr;

            for (int i = ; i < path.size()-; ++i)

                sstr << path[i]<<"->";

            sstr<<path[path.size()-];

            result.push_back(sstr.str());

        }

        else

        {

            if (root->left)

                allPath(root->left, result, path);

            if (root->right)

                allPath(root->right, result, path);

        }

        path.pop_back();

    }

    vector<string> binaryTreePaths(TreeNode* root) {

        vector<string>result;

        vector<int>path;

        allPath(root, result,path);

        return result;

    }