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.

Example

Given [1,2,3,4,5,6,7], return

     4
   /     2     6
 / \    / 1   3  5   7

分析：这是一道非常明显的递归题。取array的中间数作为树的root，array 左边部分是左子树部分，array右边部分是右子树部分。

 1 /**
 2  * Definition of TreeNode:
 3  * public class TreeNode {
 4  *     public int val;
 5  *     public TreeNode left, right;
 6  *     public TreeNode(int val) {
 7  *         this.val = val;
 8  *         this.left = this.right = null;
 9  *     }
10  * }
11  */
12 public class Solution {
13     /**
14      * @param A: an integer array
15      * @return: a tree node
16      * cnblogs.com/beiyeqingteng/
17      */
18     public TreeNode sortedArrayToBST(int[] A) {
19         if (A == null || A.length == 0) return null;
20         return helper(A, 0, A.length - 1);
21     }
22
23     public TreeNode helper(int[] A, int i, int j) {
24         if (i == j) {
25             return new TreeNode(A[i]);
26         } else if (i > j) {
27             return null;
28         }  else {
29             int mid = i + (j - i) / 2;
30             TreeNode root = new TreeNode(A[mid]);
31             root.left = helper(A, i, mid - 1);
32             root.right = helper(A, mid + 1, j);
33             return root;
34         }
35     }
36 }

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