Fix threeSum.cpp.

1. Use burte-force, O(n^3).
2. optimize with map, O(n^2).
3. optimize with hash_map, O(n^2).
4. optimize with new algorithm, maybe O(n^2).
5. optimize with another new algorithm, O(n^2).
6. implemented a hash-map, need debug.

Signed-off-by: Wayne Wang <wythern@gmail.com>
Add more accepted answers.

Signed-off-by: Wayne Wang <wythern@gmail.com>
Update common.h for better use.

Signed-off-by: Wayne Wang <wythern@gmail.com>
Fix conflicts.

Signed-off-by: Wayne Wang <wythern@gmail.com>