leetcode 66: Convert Sorted List to Binary Search Tree

Convert Sorted List to Binary Search TreeOct 3 '12

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

uncompleted

 

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; next = null; }
 * }
 */
/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public TreeNode sortedListToBST(ListNode head) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if( head==null ) return null;
        ListNode p = head;
        while(p.next!=null) {
            p = p.next;
        }
        return bstRec(head, p);
    }
    
    private TreeNode bstRec(ListNode low, ListNode high) {
        if(low == high) return null;
        ListNode p=low, q=low;
        
        while(q!=high) {
            if( q.next == null ) break;
            q = q.next.next;
            p = p.next;
        }
        
        TreeNode head = new TreeNode( p.next.val );
        head.left = bstRec(low, p);
        
        if(p.next!=null){
            bstRec(p.next,high);
        }
        
        return head;
    }
}