leetcode 11: 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 *sortedArrayToBST(vector<int> &num) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if( num.size() <= 0) return NULL;
        
        int size = num.size();
        
        return makeSubtree(num, 0, size-1); 
    }
    
    TreeNode * makeSubtree( vector<int> &num, int low , int high) {
        if(low>high) return NULL;
        
        
        
        int mid = (low + high) / 2;
        TreeNode * root = new TreeNode( num[mid]);
        
        root->left = makeSubtree( num, low, mid-1);
        root->right = makeSubtree( num, mid+1, high);
        
        return root;
    }
};