[Linked List]Convert Sorted List to Binary Search Tree

Total Accepted: 57775 Total Submissions: 198972 Difficulty: Medium

 

Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.

 (M) Convert Sorted Array to Binary Search Tree

 

/**
 * 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:
    int getListNodeLength(ListNode* head)
    {
        int len = 0;
        while(head){
            len++;
            head = head->next;
        }
        return len;
    }
    TreeNode* sortedListToBST(ListNode* head) {
        if(head==NULL){
            return NULL;
        }
        if(head->next==NULL){
            return new TreeNode(head->val);
        }
        int len = getListNodeLength(head);
        int cnt = len/2;
     
        ListNode* cur      = head;
        while(--cnt){
            cur = cur->next;
        }
        ListNode* mid  = cur->next;
        ListNode* head2= mid->next;
        cur->next      = NULL;
        mid->next      = NULL;

        
        TreeNode* root = new TreeNode(mid->val);
        root->left     = sortedListToBST(head);
        root->right    = sortedListToBST(head2);
        return root;
    }
};

Next challenges: (M) Convert Sorted Array to Binary Search Tree