109.) Convert Sorted List to Binary Search Tree.cpp

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    TreeNode* sortedListToBST(ListNode* head) {
        if(!head)
            return NULL;
        if(!head->next)
            return new TreeNode(head->val);

        ListNode* p = head;
        ListNode* q = head;
        ListNode* r = NULL;

        while(p->next)
        {
            p = p->next;
            if(p->next)
                p = p->next;
            r = q;
            q = q->next;
        }
        TreeNode* root = new TreeNode(q->val);
        r->next = NULL;
        root->left = sortedListToBST(head);
        root->right = sortedListToBST(q->next);

        return root;
    }
};