108. Convert Sorted Array to Bin

注意BST的格式，如何建立BST等的知识

/**
 * 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.length == 0){
            return null;
        }
        if(nums.length == 1){
            return new TreeNode(nums[0]);
        }
        int size = nums.length;
        TreeNode root = helper(nums, 0, size-1);
        return root;
    }
    // 选定了helper函数返回TreeNode之后逻辑就非常清楚了
    private TreeNode helper(int[] nums, int l, int r){
        int length = r-l+1;
        int cutPoint = l + length/2;
        if(length == 1){
            return new TreeNode(nums[l]);
        }
        if(length < 1){
            return null;
        }
        TreeNode node = new TreeNode(nums[cutPoint]);
        node.left = helper(nums, l, cutPoint-1);
        node.right = helper(nums, cutPoint+1, r);
        return node;
    }
}