108. 将有序数组转换为二叉搜索树

题目链接：https://leetcode-cn.com/problems/convert-sorted-array-to-binary-search-tree/submissions/

解题思路：

这个题有点类似二分查找，根节点在正中间，左子树的根节点在左边一半数组的正中间，右子树的根节点在右边一半数组的正中间；于是可以递归解决：先确定根节点，然后构造左子树，然后构造右子树， 结束条件类似二分查找(low > high)。

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