Added uppercase/lowercase string and check for substrings
Added uppercase/lowercase string and check for substrings

nguyenantinhbk77/practice-swift,hanhailong/practice-swift,hanhailong/practice-swift,domenicosolazzo/practice-swift,domenicosolazzo/practice-swift,domenicosolazzo/practice-swift,nguyenantinhbk77/practice-swift,nguyenantinhbk77/practice-swift,hanhailong/practice-swift,domenicosolazzo/practice-swift,nguyenantinhbk77/practice-swift,hanhailong/practice-swift