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.

» Solve this problem

Complexity: O(n^2)

/**
 * 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:
    TreeNode *sortedListToBST(ListNode *head) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        int n = 0;
        ListNode *node = head;
        while (node) {
            ++n;
            node = node->next;
        }
        return toBST(head, n);
    }
    
    // n - the total number of nodes needs processing
    TreeNode *toBST(ListNode *head, int n) {
        if (!head || n <= 0) return NULL;
        
        int mid = n / 2;
        ListNode *midNode = head;
        for (int i = 0; i < mid; ++i) {
            midNode = midNode->next;
        }
        
        TreeNode *root = new TreeNode(midNode->val);
        root->left = toBST(head, mid);
        root->right = toBST(midNode->next, n - mid - 1);
        return root;
    }
};