Convert Sorted Array to Binary Search Tree with Minimal Height

题目：

Given a sorted (increasing order) array, Convert it to create a binary tree with minimal height.

分析：

基本题~

解法：

public class Solution {
    /**
     * @param A: an integer array
     * @return: a tree node
     */
    public TreeNode helper(int[] A, int start, int end) {

        int mid = start + (end - start) / 2;
        TreeNode root = new TreeNode(A[mid]);

        if (start <= mid - 1) {
            root.left = helper(A, start, mid - 1);
        }

        if (mid + 1 <= end) {
            root.right = helper(A, mid + 1, end);
        }
        return root;
    } 

    public TreeNode sortedArrayToBST(int[] A) {  
        // write your code here
        if (A.length == 0) {
            return null;
        }
        return helper(A, 0, A.length - 1);
    }  
}