Flatten Binary Tree to Linked List

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */
void flatten(struct TreeNode* root) {
    struct TreeNode *tmp = NULL, *next = NULL;
    if(root == NULL)
        return NULL;
    if(root->left != NULL)
    {
        tmp = root->right;
        root->right = root->left;
        flatten(root->left);
        root->left = NULL;
        while(root->right != NULL)
            root = root->right;
        root->right = tmp;
        flatten(tmp);
    }
    else
        flatten(root->right);
}