108. Convert Sorted Array to Binary Search Tree

preorder O(n) time

/**
 * 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) {
        return sortedArrayToBST(begin(nums), end(nums));
    }

private:
    typedef vector<int>::iterator Iter;

    TreeNode *sortedArrayToBST(Iter b, Iter e) {
        if (b == e) return nullptr;
        auto m = b + (e - b) / 2;
        auto res = new TreeNode(*m);
        res->left = sortedArrayToBST(b, m);
        res->right = sortedArrayToBST(m + 1, e);
        return res;
    }
};