数据结构练习（01）把二元查找树转变成排序的双向链表

struct tree {
int value;
tree *left, *right;
};

void convert(tree *&head, tree *&tail, tree *root)
{
if (!root)  {
head = tail = NULL;
return ;
}

tree *templeft, *tempright;

templeft = tempright = NULL;

convert(head, templeft, root->left);
convert(tempright, tail, root->right);

if (templeft) {
templeft->right = root;
root->left = templeft;
}
else {
head = root;
}

if (tempright) {
tempright->left = root;
root->right = tempright;
}
else {
tail = root;
}
}

tree* tree2link(tree *root)
{
tree *head, *tail;
convert(head, tail, root);
　　 return head;
}