递归---Convert Sorted Array to Binary Search Tree With Minimal Height

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

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Example
Given [1,2,3,4,5,6,7], return

4
/ \
2 6
/ \ / \
1 3 5 7
Note
There may exist multiple valid solutions, return any of them.

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/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */ 
public class Solution {
    /**
     * @param A: an integer array
     * @return: a tree node
     */
      private TreeNode buildTree(int[] num, int start, int end) {
        if (start > end) {
            return null;
        }

        TreeNode node = new TreeNode(num[(start + end) / 2]);
        node.left = buildTree(num, start, (start + end) / 2 - 1);
        node.right = buildTree(num, (start + end) / 2 + 1, end);
        return node;
    }

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

    }  
}