177. Convert Sorted Array to Binary Search Tree With Minimal Height【LintCode by java】

Description

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

There may exist multiple valid solutions, return any of them.

Example

Given [1,2,3,4,5,6,7], return

     4
   /   \
  2     6
 / \    / \
1   3  5   7

解题：题目要求根据一个有序的数组创建一个二叉排序树，并且返回这个排序树。要求高度最小，那么必定是平衡二叉树，每次取有序序列最中间的那个数作为结点即可。递归方法，代码如下：

 /**
  * 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
         return create(A, 0, A.length - 1);
     }
     private TreeNode create(int[]A, int first, int last){
         if(first >last)
            return null;
         int mid = (first + last) / 2;
         TreeNode node = new TreeNode( A[mid] );
         node.left = create(A, first, mid-1);
         node.right = create(A, mid+1, last);
         return node;
     }
 }