CareerCup

node *bstree::createTree(int arr[], int start, int end){
    
    if(start > end)
        return NULL;
    int middle = (start + end)/2;
    node *root = createRoot(arr[middle]);
    root-> left = createTree(arr, start, middle-1);
    root-> right = createTree(arr, middle+1, end);
    
    return root;
}

1. Insert middle of array as root.
2. recurse left getting middle of left section, after setting array size to start to middle-1(from step 1).
3. recurse right after setting array to middle+1 to end.

node *bstree::createTree(int arr[], int start, int end){
    
    if(start > end)
        return NULL;
    int middle = (start + end)/2;
    node *root = createRoot(arr[middle]);
    root-> left = createTree(arr, start, middle-1);
    root-> right = createTree(arr, middle+1, end);
    
    return root;
}

Working code :: 

class Node{
        Node left, right;
        int data;
         Node(int data1){
            this.data = data1;
            left = null;
            right = null;
        }
          
    }
public class sortedArrayToBinaryTree {
   
    public Node convert(int[] arr, int start, int end){
        if(start > end) {
            return null;
        }
        int mid = (start+end)/2;
        Node root = new Node(arr[mid]);
        root.left = convert(arr, start, mid-1);
        root.right = convert(arr, mid+1, end);
        
        return root;        
    }
    
    public void display(Node root){
        
        if(root != null) {
            display(root.left);
                System.out.println(root.data);
            display(root.right);
        }
    }
    
    
    public static void main(String[] args){
        sortedArrayToBinaryTree s = new sortedArrayToBinaryTree();
        int[] arr = {1,2,3,4,5,6,7,8,9,10};
        Node n = s.convert(arr, 0 , arr.length-1);
        System.out.println("Display Tree");
        s.display(n);
    }    
}

public TreeNode toBST(int[] arr){
		return toBSTHelper(arr, 0, arr.length - 1);
	}
	
	public TreeNode toBSTHelper(int[] arr, int low, int high){
		if(low > high)
			return null;
		int middle = (low + high)/2;
		TreeNode root = new TreeNode(arr[middle]);
		root.left = toBSTHelper(arr, low, middle-1);
		root.right = toBSTHelper(arr, middle+1, high);
		return root;
	}