LeetCode 114. Flatten Binary Tree to Linked List

class Solution {
public:
    void flatten(TreeNode* root) {
        if(root==NULL) return;
        dfs(root,root->left,root->right);  
        root->left=NULL;
        
    }
    
    void dfs(TreeNode* root,TreeNode* a,TreeNode* b)
    {
        flatten(a);
        if(root->right==NULL)
        {
            root->right = a;
            return;
        }
        if(root->right!=NULL)
        {
            flatten(b);
            if(a!=NULL)
            {
            join(a,b);
           root->right = a; 
            }
        }
       
    }
    void join(TreeNode *a,TreeNode *b)
    {
        if(a->right!=NULL)
            join(a->right,b);
        else
        {
         a->right = b;
        }
    }
    
    
};