CC4.3.java

    public TreeNode buildTree(int[] A) {
        return buildBST(A, 0, A.length-1);
    }
    
    private TreeNode buildBST(int[] A, int left, int right) {
        TreeNode root = null;
        if (left <= right) {
            int mid = left + (right - left) / 2;
            root = new TreeNode(A[mid]);
            root.left = buildBST(A, left, mid-1);
            root.right = buildBST(A, mid+1, right);
        }
        return root;
    }