0108 - Convert Sorted Arary to Binary Search Tree

0108 - Convert Sorted Arary to Binary Search Tree

Given an integer array nums where the elements are sorted in ascending order, convert it to a height-balanced binary search tree.

A height-balanced binary tree is a binary tree in which the depth of the two subtrees of every node never differs by more than one.

Examples

Input: nums = [-10,-3,0,5,9] Output: [0,-3,9,-10,null,5] Explanation: [0,-10,5,null,-3,null,9] is also accepted:

Input: nums = [1,3] Output: [3,1] Explanation: [1,3] and [3,1] are both a height-balanced BSTs.

Constraints

1 <= nums.length <= 104 -104 <= nums[i] <= 104 nums is sorted in a strictly increasing order.

Java Solution

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public TreeNode sortedArrayToBST(int[] nums) {
        return helper(0, nums.length-1, nums) ;
    }

    public TreeNode helper(int left, int right, int[] nums) {
        if(left > right) return null;

        int middle = (left + right) / 2;
        TreeNode root = new TreeNode(nums[middle]);
        root.left = helper(left, middle - 1, nums);
        root.right = helper(middle + 1, right, nums);

        return root;
    }
}