Convert Sorted List to Binary Search Tree. Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.

linked_list_to_BST.cpp

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode *sortedListToBST(ListNode *head) {
        if (!head)
            return NULL;
        //find middle to be root, then left(head), and right (root->next)
        //Note: must cut off the left part, otherwise it's dead loop
        ListNode *slow = head;
        ListNode *prev = NULL;
        ListNode *fast = slow->next;            //1 -> 2: slow=1, fast=2, s=2, f=NULL;  1234: s=1, f=2; s=2; f=3, f=4
        while(fast) {
            prev = slow;
            slow = slow->next;
            fast = fast->next;
            if (!fast)
                break;
            fast = fast->next;
        }
        TreeNode *root = new TreeNode (slow->val);
        
        //cut off left part for left Child
        if (prev)
            prev->next = NULL;
        if (slow == head)
            root->left = NULL;
        else
            root->left = sortedListToBST(head);
        root->right = sortedListToBST(slow->next);
        return root;
    }
};