二叉搜索树转换成双向链表

 1 void BST2DoubleList(TreeNode *root, TreeNode *& prev, TreeNode *& head) {
 2   if (root == NULL)
 3     return;
 4   BST2DoubleList(root->left, prev, head);
 5   root->left = prev;
 6   if (prev != NULL)
 7     prev->right = root;
 8   else
 9     head = root;
10   prev = root;
11   BST2DoubleList(root->right, prev, head);
12 }
13 
14 TreeNode* treeToDoublyList(TreeNode *root) {
15   TreeNode *prev = NULL;
16   TreeNode *head = NULL;
17   BST2DoubleList(root, prev, head);
18   return head;
19 }