把二叉搜索树转换为累加树

class Solution {
public:
    int sum=0;
    TreeNode* travel(TreeNode*root){
        TreeNode* root1= new TreeNode();
        if(root==NULL) return NULL;
        if(root->right) root1->right=travel(root->right);
        sum+=root->val;
        root1->val=sum;
        if(root->left) root1->left=travel(root->left);
        return root1;
    }
    TreeNode* convertBST(TreeNode* root) {
        return travel(root);
    }
};