[leetcode]Convert Sorted Array to Binary Search Tree

Given an array where elements are sorted in ascending order, convert it to a height balanced BST.

/** * 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 *dfs(vector<int>&num, int start, int N){        if(N == 0) return NULL;        if(N == 1){            TreeNode *root = new TreeNode(num[start]);            return root;        }                int mid = start + N/2;                TreeNode *root = new TreeNode(num[mid]);        root -> left = dfs(num, start, N/2);        root -> right = dfs(num, mid+1, (N-1)/2);        return root;    }    TreeNode *sortedArrayToBST(vector<int> &num) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        if(num.empty()) return NULL;        return dfs(num, 0, num.size());            }};