tidb section2, test case false positive

[schneiderlin]

I encounter a test failure like this

➜  mapreduce git:(master) ✗ make test_homework 
go test -v -run=TestURLTop
=== RUN   TestURLTop
Case0 PASS, dataSize=1MB, nMapFiles=5, cost=46.062897ms
    TestURLTop: urltop10_test.go:89: Case1 FAIL, dataSize=1MB, nMapFiles=5, cost=20.123106ms
        expected:
        github.com/pingcap/tidb/issues/0: 5204
        github.com/pingcap/tidb/issues/3: 5189
        github.com/pingcap/tidb/issues/4: 5189
        github.com/pingcap/tidb/pull/2: 5181
        github.com/pingcap/tidb/issues/5: 5144
        github.com/pingcap/tidb/6: 5130
        github.com/pingcap/tidb/1: 5118
        
        , but got:
        github.com/pingcap/tidb/issues/0: 5204
        github.com/pingcap/tidb/issues/4: 5189
        github.com/pingcap/tidb/issues/3: 5189
        github.com/pingcap/tidb/pull/2: 5181
        github.com/pingcap/tidb/issues/5: 5144
        github.com/pingcap/tidb/6: 5130
        github.com/pingcap/tidb/1: 5118
        
        
        
--- FAIL: TestURLTop (0.08s)
FAIL
exit status 1
FAIL    talent  0.081s
Makefile:9: recipe for target 'test_homework' failed
make: *** [test_homework] Error 1

i think order of the url shouldn't matter, given they have the same frequency(5189 in this case),these two output are equivalent and test should pass.