方法:递归，对不同的数组取中点作为二叉树的头节点

    1. class Solution {
    2. public:
    3.     TreeNode* sortedArrayToBST(vector<int>& nums) {
    4.         return creatbst(nums,0,nums.size()-1);
    5.     }
    6.     TreeNode* creatbst(vector<int>& nums,int left,int right){
    7.         if(left>right){
    8.             return nullptr;
    9.         }
    10.         int mid=(right+left)/2;
    11.         TreeNode* root=new TreeNode(nums[mid]);
    12.         root->left=creatbst(nums,left,mid-1);
    13.         root->right=creatbst(nums,mid+1,right);
    14.         return root;
    15.     }
    16. };