// Builds a BST from the (s + 1)-th to the e-th node in L, and returns the
// root. Node numbering is from 1 to n.
TreeNode * BuildBSTFromSortedDoublyListHelper(ListNode **L, int s, int e) {
if (s >= e) {
return nullptr;
}
int m = s + ((e - s) / 2);
TreeNode *left = BuildBSTFromSortedDoublyListHelper(L, s, m);
TreeNode *curr = new TreeNode((*L)->val); // The last function call sets L to the successor of the
// maximum node in the tree rooted at left.
*L = (*L)->next;
curr->left = left;
curr->right = BuildBSTFromSortedDoublyListHelper(L, m + 1, e);
return curr;
}
/**
* @param head: The first node of linked list.
* @return: a tree node
*/
TreeNode *sortedListToBST(ListNode *L) {
ListNode *curr = L;
int n = 0;
while (curr) {
curr = curr->next;
++n;
}
return BuildBSTFromSortedDoublyListHelper(&L, 0, n);
}
