[LeetCode] Convert Sorted List to Binary Search Tree

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.

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* convert(ListNode *&head, int len) {
        if (len <= 0) return NULL;
        TreeNode *left = convert(head, len / 2);
        TreeNode *root = new TreeNode(head->val);
        root->left = left;
        head = head->next;
        root->right = convert(head, len - 1 - len / 2);
        return root;
    }
    TreeNode* sortedListToBST(ListNode* head) {
        ListNode *p = head;
        int len = 0;
        while (p != NULL) {
            ++len;
            p = p->next;
        }
        return convert(head, len);
    }
};