把排序数组转换为高度最小的二叉搜索树convert-sorted-array-to-binary-search-tree-with-minimal-height

给一个排序数组（从小到大），将其转换为一棵高度最小的排序二叉树。

样例

给出数组 [1,2,3,4,5,6,7], 返回

     4
   /   
  2     6
 /     / 
1   3  5   7

/**
 * 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
     */
    public TreeNode sortedArrayToBST(int[] A) {  
        // write your code here
        if(A.length ==0 )  return null;
        return sortedToBST(A, 0, A.length-1);
    }
    public TreeNode sortedToBST(int[] A,int start,int end){
        if (start > end)
            return null;
        int mid = (start+end)/2;
        TreeNode root = new TreeNode(A[mid]);
        root.left = sortedToBST(A,start,mid-1);
        root.right  = sortedToBST(A,mid+1,end);
        return root;
    }
}