LeetCode -- Convert Sorted Array to Binary Search Tree

题目描述：
Given an array where elements are sorted in ascending order, convert it to a height balanced BST.

就是对一个排序的序列生成高度平衡的二叉树。

思路：
本题的算法思路属于分治。

即在数组nums取中间元素nums[m]并生成节点添加到树上，然后对左右分别做DFS。即使用m左边元素生成左子树，使用m右边元素生成右子树。

实现代码：

/**
* Definition for a binary tree node.
* public class TreeNode {
*     public int val;
*     public TreeNode left;
*     public TreeNode right;
*     public TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public TreeNode SortedArrayToBST(int[] nums)
{
if(nums == null || nums.Length == 0){
return null;
}
var node = new TreeNode(nums[nums.Length/2]);
BuildTree(nums, 0, nums.Length - 1, ref node);
return node;
}

private void BuildTree(int[] nums,int start, int end, ref TreeNode current)
{
if(start > end){
return ;
}

var m = (start + end) / 2;
current = new TreeNode(nums[m]);
if(start == end){
return;
}

BuildTree(nums, start, m - 1, ref current.left);
BuildTree(nums, m + 1, end, ref current.right);
}

}