题解 | #将升序数组转化为平衡二叉搜索树#

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 * }
 */

public class Solution {
    /**
     * 
     * @param num int整型一维数组 
     * @return TreeNode类
     */
    public TreeNode sortedArrayToBST (int[] num) {
        // write code here
        if(num == null ||num.length == 0){
            return null;
        }
        //选择区间中点作为根节点
        return createBST(num, 0 , num.length - 1);
        
    }
    private TreeNode createBST(int[] num, int left, int right){
        //递归终止 
        if(left >right) return null;
        int mid = (right - left + 1) / 2  +left;
        TreeNode root = new TreeNode(num[mid]);
        root.left = createBST(num, left,mid - 1);
        root.right = createBST(num, mid +1,right);
        return root;
    }
}