Flatten Binary Trees to Linked Lists

class Solution {
public:
    void flatten(TreeNode* root) {
        preorder(root);
    }
private:
    void preorder(TreeNode *node) {
        if (node == NULL) return;
        preorder(node->right);
        preorder(node->left);
        node->right = pre;
        node->left = NULL;
        pre = node;
    }
    TreeNode* pre=NULL;
};