[LeetCode] Convert Sorted Array to Binary Search Tree

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public TreeNode sortedArrayToBST(int[] num) {
        return sortedArrayToBSTHelper(num, 0, num.length);
    }
    
    public TreeNode sortedArrayToBSTHelper(int[] num, int startIndex, int length) {
        if (length <= 0) return null;
        
        int rootIndex = startIndex + length/2;
        int leftLength = rootIndex - startIndex;
        int rightLength = length - leftLength - 1;
        TreeNode root = new TreeNode(num[rootIndex]);
        root.left = sortedArrayToBSTHelper(num, startIndex, leftLength);
        root.right = sortedArrayToBSTHelper(num, rootIndex+1, rightLength);
        
        return root;
    }
}