Convert Sorted Array to Binary Search Tree

题目：

Given an array where elements are sorted in ascending order, convert it to BST

解答：

public class Solution {

    public static void main(String[] args) {
        int[] num = {1,2,3,4,5};

        TreeNode root = sortedArrayToBST(num);

        InOrder(root);

    }

    public static TreeNode sortedArrayToBST(int[] num) {
        return sortedArrayToBST(num, 0, num.length-1);
    }

    private static sortedArrayToBST(int[] arr, int start, int end) {
        if(start > end) {
            return null;
        }

        int mid = (start + end) >> 1;
        TreeNode node = new TreeNode(arr[mid]);
        node.left = sortedArrayToBST(arr, start, mid-1);
        node.right = sortedArrayToBST(arr, mid+1, end);
        return node;
    }

    public static void InOrder(TreeNode root) {

        if(root == null) {
            return;
        }

        if(root.left != null) {
            InOrder(root.left);
        }

        System.out.println(root.val);

        if(root.right != null) {
            InOrder(root.right);
        }

    }
}