https://leetcode.com/problems/convert-sorted-array-to-binary-search-tree/description/ Given an array where elements are sorted in ascending order, convert it to a height balanced BST. For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees of every node never differ by more than 1. Example: Given the sorted array: [-10,-3,0,5,9], One possible answer is: [0,-3,9,-10,null,5], which represents the following height balanced BST:       0      / \    -3   9    /   /  -10  5 time: o(n) space: o(height) pre order: 先弄自己， 再管下面的兄弟

public TreeNode sortedArrayToBST(int[] nums) {        if (nums  == null || nums.length ==0 )return null ;        int left = 0, right = nums.length -1 ;        return helper(nums, left, right);    }    //everytime create the root and connect to left and right    private TreeNode helper(int[] nums, int left, int right) {        int mid = left + (right - left) /2 ;        TreeNode node = new TreeNode(nums[mid]) ;        if (mid+ 1<=right){            node.right = helper(nums, mid+1, right) ;        }        if (mid -1 >= left){            node.left = helper(nums, left, mid-1) ;        }        //going down and repeat        return node ;    }