Approve cpplint

except #include <algorithm> (which is not necessary)
Fix typo

this commit is unconcerned from current branch,
but it's too trivial to make a topic branch.
Add `// NOLINT'
Refactor sort_and_merge
Fix sort_and_merge

Assignment is better than swap in this case
Optimize sort_and_merge futher

If vector's size is 1, no process is required.
Merge pull request #371 from gintenlabo/refactor-sort_and_merge

Refactor sort_and_merge
Merge pull request #368 from gintenlabo/fix-cpplint-violations

Fix cpplint violations