Convert Sorted Array to Binary Search Tree

Problem:

Given an array where elements are sorted in ascending order, convert it to a height balanced BST.

先序遍历。

Solution:

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    int[] gnum;
public TreeNode sortedArrayToBST(int[] num) {
        gnum = num;
        return subBST(0, num.length);
    }
    
    private TreeNode subBST(int start,int len)
    {
         if(len==0)
             return null;
    
         TreeNode root = new TreeNode(0);
    
         root.val = gnum[start+len/2];
         root.left = subBST(start, len/2);
         root.right = subBST(start+len/2+1, (len-1)/2);
         return root;
    }
}