assertEqualCharNodes(traverser.breadthFirst(charactersOf("bd")), "bdca");
    assertEqualCharNodes(traverser.breadthFirst(charactersOf("dc")), "dcab");
    assertEqualCharNodes(traverser.breadthFirst(charactersOf("bc")), "bcda");
  }

  @Test
  public void forGraph_breadthFirst_twoCycles() {
    Traverser<Character> traverser = Traverser.forGraph(TWO_CYCLES_GRAPH);

    assertEqualCharNodes(traverser.breadthFirst(charactersOf("bd")), "bdca");
    assertEqualCharNodes(traverser.breadthFirst(charactersOf("dc")), "dcab");
    assertEqualCharNodes(traverser.breadthFirst(charactersOf("bc")), "bcda");
  }

  @Test
  public void forGraph_breadthFirst_twoCycles() {
    Traverser<Character> traverser = Traverser.forGraph(TWO_CYCLES_GRAPH);

    // Two orphan low surrogates.
    assertEquals("\uDCAB", Strings.commonSuffix("\uDCAB", "\uDCAB"));
  }

  public void testValidSurrogatePairAt() {
    assertTrue(Strings.validSurrogatePairAt("\uD8AB\uDCAB", 0));
    assertTrue(Strings.validSurrogatePairAt("abc\uD8AB\uDCAB", 3));
    assertTrue(Strings.validSurrogatePairAt("abc\uD8AB\uDCABxyz", 3));

    // Two orphan low surrogates.
    assertEquals("\uDCAB", Strings.commonSuffix("\uDCAB", "\uDCAB"));
  }

  public void testValidSurrogatePairAt() {
    assertTrue(Strings.validSurrogatePairAt("\uD8AB\uDCAB", 0));
    assertTrue(Strings.validSurrogatePairAt("abc\uD8AB\uDCAB", 3));
    assertTrue(Strings.validSurrogatePairAt("abc\uD8AB\uDCABxyz", 3));

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