Convert Sorted Array to Binary Search Tree

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

Solution: Recursion.

/**
 * 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) {
        return buildBST(num, 0, num.size()-1);
    }
    
    TreeNode* buildBST(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 = buildBST(num, start, mid-1);
        root->right = buildBST(num, mid+1, end);
        return root;
    }
};