有序数组构建平衡二叉树

class Solution {//递归解决
        public TreeNode sortedArrayToBST(int[] num) {
        if (num.length == 0)
            return null;
        return sortedArrayToBST(num, 0, num.length - 1);
    }

    public TreeNode sortedArrayToBST(int[] num, int start, int end) {
        if (start > end)
            return null;
        int mid = (start + end) >> 1;//即除2
        TreeNode root = new TreeNode(num[mid]);
        root.left = sortedArrayToBST(num, start, mid - 1);
        root.right = sortedArrayToBST(num, mid + 1, end);
        return root;
    }

}