if at any point one root becomes null and the other root is not null , then they are not mirrors . This condition verifies that structures of tree are mirrors of each other .
if ( ! ( root1 == null && root2 == null ) ) return " No " ; if ( st1 . Count != 0 && st2 . Count != 0 ) { root1 = st1 . Peek ( ) ; root2 = st2 . Peek ( ) ; st1 . Pop ( ) ; st2 . Pop ( ) ;