public static void main(String[] argvs) {
   MinimumWindowSubstring mws = new MinimumWindowSubstring();
   System.out.println(mws.minWindow("bba", "ab"));
 }
 @Test
 public void testNormalCases() {
   Assert.assertEquals(MinimumWindowSubstring.minWindow("BANCFBD", "ABC"), "BANC");
 }
 @Test
 public void testEmptyString() {
   Assert.assertEquals(MinimumWindowSubstring.minWindow("", ""), "");
 }
 @Test
 public void testLongerTargetString() {
   Assert.assertEquals(MinimumWindowSubstring.minWindow("abc", "abcd"), "");
 }