5.3.4 Convert Sorted Array to Binary Searh Tree

Notes:
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) / 2;
TreeNode *root = new TreeNode(num[mid]);
root->left = buildBST(num, start, mid - 1);
root->right = buildBST(num, mid + 1, end);

return root;
}
};