[LeetCode] 108. Convert Sorted Array to Binary Search Tree Java

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

题意及分析：给出一个有序序列，构造一棵平衡二叉查找树。二叉查找树：对于任意一个节点，都有左子节点小于右节点；二叉平衡树：对于任意一个节点，左右子树的高度差不超过1。这里可以将序列的中点当做根节点，然后递归左右子序列。

代码：

/**
 * 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) {
        return bulidTree(nums,0,nums.length-1);
    }

    public TreeNode bulidTree(int[] nums,int start,int end){
        if(start>end) return null;
        int mid = (start+end)/2;
        TreeNode root = new TreeNode(nums[mid]);
        root.left = bulidTree(nums,start,mid-1);
        root.right = bulidTree(nums,mid+1,end);
        return root;
    }
}