Fix cpplint errors in paddle/fluid/string (#9667)

* Fix cpplint errors in paddle/fluid/string

* Fix unit test error

* Correct

Author: wangkuiyi

paddle/fluid/string/piece.cc

@@ -12,7 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#include "piece.h"
+#include "paddle/fluid/string/piece.h"
 
 #include <string.h>
 

paddle/fluid/string/printf.h

@@ -71,6 +71,8 @@
 
 #include <iostream>
 #include <sstream>
+#include <string>
+
 #include "tinyformat/tinyformat.h"  // https://github.com/c42f/tinyformat
 
 namespace paddle {

paddle/fluid/string/printf_test.cc

@@ -11,7 +11,8 @@
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.
-#include "printf.h"
+
+#include "paddle/fluid/string/printf.h"
 
 #include <string>
 
@@ -21,7 +22,7 @@ TEST(StringPrintf, StringPrintf) {
   std::string weekday = "Wednesday";
   const char* month = "July";
   size_t day = 27;
-  long hour = 14;
+  int hour = 14;
   int min = 44;
   EXPECT_EQ(std::string("Wednesday, July 27, 14:44"),
             paddle::string::Sprintf("%s, %s %d, %.2d:%.2d", weekday, month, day,

paddle/fluid/string/to_string_test.cc

@@ -12,7 +12,7 @@ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 See the License for the specific language governing permissions and
 limitations under the License. */
 
-#include "to_string.h"
+#include "paddle/fluid/string/to_string.h"
 #include <gtest/gtest.h>
 
 constexpr char kOutputString[] = "User Defined Output";
@@ -26,14 +26,13 @@ std::ostream& operator<<(std::ostream& s, const UserDefinedClass& ins) {
 }
 
 TEST(to_string, normal) {
-  using namespace paddle::string;
+  using paddle::string::to_string;
   ASSERT_EQ("10", to_string(10));
   ASSERT_EQ("abc", to_string("abc"));
   ASSERT_EQ("1.2", to_string(1.2));
 }
 
 TEST(to_string, user_defined) {
-  using namespace paddle::string;
   UserDefinedClass instance;
-  ASSERT_EQ(kOutputString, to_string(instance));
+  ASSERT_EQ(kOutputString, paddle::string::to_string(instance));
 }