Leetcode - Convert Sorted Array to Binary Search Tree

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
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public TreeNode sortedArrayToBST(int[] num) {
        if(num == null || num.length == 0) return null;
        
        int start = 0, end = num.length - 1;

        TreeNode root = buildTree(num, start, end);
        
        return root;
    }
    
    public TreeNode buildTree(int[] num, int start, int end) {
        if(start > end) return null;
        
        int mid = (start + end) / 2;
        
        // build left sub tree
        TreeNode left = buildTree(num, start, mid - 1);
        // build root of the subtree
        TreeNode node = new TreeNode(num[mid]);
        node.left = left;
        // build right sub tree
        node.right = buildTree(num, mid + 1, end);
        
        return node;
    }
}