解决思路

class Solution {
  int[] nums;
  public TreeNode helper(int left, int right) {
    if (left > right) return null;
    // always choose left middle node as a root
    int p = (left + right) / 2;
    // inorder traversal: left -> node -> right
    TreeNode root = new TreeNode(nums[p]);
    root.left = helper(left, p - 1);
    root.right = helper(p + 1, right);
    return root;
  }
  public TreeNode sortedArrayToBST(int[] nums) {
    this.nums = nums;
    return helper(0, nums.length - 1);
  }
}