[108. Convert Sorted Array to Binary Search Tree] #leetcode

108_0806.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public TreeNode sortedArrayToBST(int[] nums) {
        if (nums.length == 0) return null;
        return helper(nums, 0, nums.length-1);
    }
    
    public TreeNode helper(int[] nums, int start, int end) {
        if (start > end) return null;
        
        int mid = (end - start) / 2 + start;
        TreeNode left = helper(nums, start, mid-1);
        TreeNode right = helper(nums, mid+1, end);
        
        TreeNode node = new TreeNode(nums[mid]);
        node.left = left;
        node.right = right;
        
        return node;
    }
}