Given an array where elements are sorted in ascending order, convert it to a height balanced BST.

/** * Definition for a binary tree node. * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    TreeNode* sortedArrayToBST( vector
     
      & nums ) {                return helper( nums, 0, nums.size() - 1);    }    TreeNode* helper( vector
      
       & nums, int lefts, int rights )    {        if ( lefts > rights )               return nullptr;                       int mid = lefts + ( rights - lefts ) / 2;  // 二分法求取中间值        TreeNode* root = new TreeNode(nums[mid]); // 将中间值赋值给根节点        root -> left = helper( nums, lefts, mid -1); // 递归调用，构造左子树        root -> right = helper( nums, mid + 1, rights);// 递归， 构造右子树        return root;    }};