/**
 * 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<int>& nums) {
        if(nums.empty())return NULL;
        return BuildTree(nums,0,nums.size()-1);
    }
    TreeNode* BuildTree(vector<int>& nums,int first,int end)
    {
        if(first>end)return NULL;
        if(first==end)return new TreeNode(nums[first]);
        int mid=(first+end)/2;
        TreeNode* root=new TreeNode(nums[mid]);
        root->left=BuildTree(nums,first,mid-1);
        root->right=BuildTree(nums,mid+1,end);
        return root;
    }
};