【leetcode】Flatten Binary Tree to Linked List

TreeNode *&link(TreeNode *&root)
	{
		if(root == NULL || root->left == NULL && root->right == NULL) 
			return root;
		TreeNode *root_left = NULL;
		TreeNode *root_right = NULL;
		root_left = link(root->left);
		root_right = link(root->right);
		if(root_left != NULL)
		{
			TreeNode *tail = root_left;
			//find the last one of root->left.
			while(tail->right) tail = tail->right;
			tail->right = root_right;
		}
		else
			root_left = root_right;
		root->right = root_left;
		root->left = NULL;
		return root;
	}
	void flatten(TreeNode *root) {
		if(root == NULL) return;
		root = link(root);
	}

void preVisit(TreeNode*& pre, TreeNode*root)
	{
		if(root == NULL)
			return;
		TreeNode *right = root->right;
		TreeNode *left = root->left;
		root->left = NULL;
		if(pre == NULL)
			pre = root;
		else
		{
			pre->right = root;
			pre = root;
		}
		if(left)
			preVisit(pre, left);
		if(right)
			preVisit(pre, right);
	}

void flatten(TreeNode* root)
	{
		if(root = NULL) return;
		TreeNode *pre = NULL;
		preVisit(pre, root);
	}