/**
 * 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) {
        int n = 0;
        if(!(n = nums.size())) return nullptr;
        nums.insert(nums.begin(), 0);
        function<TreeNode*(int , int)> built = [&](int l, int r)->TreeNode* {
            if(l > r) return nullptr;
            int m = (l + r) >> 1;
            TreeNode *x = new TreeNode(nums[m]);
            x->left = built(l, m - 1);
            x->right = built(m + 1, r);
            return x;
        };
        return built(1, n);
    }
};