109 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.

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) {
        if ( head == NULL ) return NULL;
        int length = 0;
        ListNode *p = head;
        while ( p != NULL ) {
            p = p->next;
            length += 1;
        }
        TreeNode *tnode;
        if ( length == 1 ) {
            tnode = new TreeNode(head->val);
            return tnode;
        }
        int n = length/2 + 1;    // n-th node is the root
        // find (n-1)th node
        p = head;
        for ( int i = 1; i <= n-2; i++ ) p = p->next;
        ListNode *proot = p->next;
        p->next = NULL;
        tnode = new TreeNode(proot->val);
        tnode->left = sortedListToBST(head);
        tnode->right = sortedListToBST(proot->next);
        return tnode;
    }
};