Convert Sorted Array to Binary Search Tree

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

 

Code:

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/**  * 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 *res;     TreeNode *BST(vector<int> &num, int begin, int end){         if(begin>end)             return NULL;         int mid=(begin+end)/2;         TreeNode *node = new TreeNode(num[mid]);         node->left=BST(num,begin,mid-1);         node->right=BST(num,mid+1,end);         return node;     }     TreeNode *sortedArrayToBST(vector<int> &num) {         return BST(num,0,num.size()-1);     } };