TreeNode buildBST(int[] a, int start, int end) {
if (start > end) return null;
int mid = (start + end) / 2;
TreeNode n = new TreeNode(a[mid]);
n.left = buildBST(a, start, mid - 1);
n.right = buildBST(a, mid + 1, end);
return n;
}
public void invertTreeInternal(TreeNode root) {
if (root == null) return;
TreeNode temp = root.right;
root.right = root.left;
root.left = temp;
invertTree(root.left);
invertTree(root.right);
}