LeetCode_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 binary tree

 * struct TreeNode {

 *     int val;

 *     TreeNode *left;

 *     TreeNode *right;

 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}

 * };

 */

class Solution {

public:

    int countNodeNumb(ListNode *head)

    {

      int count = 0;

      while(head)

      {

        count++;

        head = head->next;

      }

      

      return count ;

    

    }

    TreeNode * BST(ListNode *head, int size)

    {

      if(size == 1) return  new TreeNode(head->val);

     

      int half = (size + 1 )/ 2;

      

      int count = 1;

      ListNode *pre, *p;

      pre = NULL;

      p = head;

      while(count < half)

      {

        pre = p;

        p = p->next;

        count ++ ;

      }

      TreeNode *Node = new TreeNode(p->val);

      

      Node->left = half-1 > 0 ? BST(head, half-1) : NULL ;

      Node->right = size - half > 0? BST(p->next, size - half) : NULL;

     

     return Node ;

    }

    TreeNode *sortedListToBST(ListNode *head) {

        // Start typing your C/C++ solution below

        // DO NOT write int main() function

        

        if(head == NULL) return NULL ;

        

        int size = countNodeNumb(head) ;

        TreeNode * myNode = BST(head, size);

        

        return myNode;

    }

};