[leetcode 108] 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) {
const int n = num.size();
if (n == 0) {
return NULL;
}
TreeNode *root;
return dfs(num,0,n);

}
TreeNode *dfs(vector<int> &num, int start, int end) {
if (start == end) {
return NULL;
}
int mid = start + (end-start)/2;
TreeNode *root = new TreeNode(num[mid]);
root->left = dfs(num, start, mid);
root->right = dfs(num, mid+1, end);
return root;
}
};