Leetcode 109 Solution

This article provides solution to leetcode question 109 (convert-sorted-list-to-binary-search-tree)
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https://leetcode.com/problems/convert-sorted-list-to-binary-search-tree

Solution

/**
 * 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* sortedListToBST(ListNode* head, int cnt) {
        if (head == NULL || cnt == 0)
            return NULL;

        ListNode* p1 = head;

        for (int i = 0; i < cnt / 2; i++)
        {
            p1 = p1->next;
        }

        TreeNode* node = new TreeNode(p1->val);
        node->left = sortedListToBST(head, cnt / 2);
        node->right = sortedListToBST(p1->next, cnt - cnt / 2 - 1);

        return node;
    }

    TreeNode* sortedListToBST(ListNode* head) {
        ListNode* p = head;

        int cnt = 0;

        while (p)
        {
            p = p->next;
            cnt++;
        }

        return sortedListToBST(head, cnt);
    }
};