Convert Sorted Array to Binary Search Tree

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode *sortedArrayToBST(vector<int> &num) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        TreeNode *t;
        int len=num.size();
        int mid=len/2;
        if(num.size()==0)
            return NULL;
        if(num.size()==1)
        {
            t=(TreeNode *)malloc(sizeof(struct TreeNode));
            t->val=num[0];
            t->left=t->right=NULL;
            return t;
        }
        t=(TreeNode *)malloc(sizeof(struct TreeNode));
        t->val=num[mid];
        vector<int>v1(num.begin(),num.begin()+mid);
        vector<int>v2(num.begin()+mid+1,num.end());
        t->left=sortedArrayToBST(v1);
        t->right=sortedArrayToBST(v2);
        return t;
    }
};