An answer of http://nabetani.sakura.ne.jp/hena/ordf03triom/

yhpg.cpp

#include <regex>
#include <string>
#include <vector>
#include <numeric>
#include <iostream>
#include "boost/algorithm/string.hpp"

class Triomino
{
    public:
        Triomino(const char name, const std::regex & pattern)
            :name_(name), pattern_(pattern)
        {}

        bool exist_in(const std::string & field) const
        {
            return std::regex_search(field, pattern_);
        }

        char name() const { return name_; }

    private:
        const char name_;
        const std::regex pattern_;
};

std::string make_field(const std::string & src)
{
    const std::vector<std::string> v = {
        "zzzzzz",
        "abcde",
        "fghij",
        "klmno",
        "pqrst",
        "uvwxy",
        "zzzzzz",
    };
    std::string field = boost::algorithm::join(v, "zz");
    for(auto && chr : src) {
        const auto pos = field.find(chr);
        if(pos != std::string::npos) {
            field.replace(pos, 1, "#");
        }
    }
    return field;
}

char solve(const std::string & src)
{
    const std::string field = make_field(src);
    const std::vector<Triomino> triominos = {
        { 'J', std::regex("#[a-z]{5}#{2}") },
        { 'L', std::regex("#[a-z]{6}#{2}") },
        { 'T', std::regex("#{2}[a-z]{6}#") },
        { 'R', std::regex("#{2}[a-z]{5}#") },
        { 'I', std::regex("#[a-z]{6}#[a-z]{6}#") },
        { 'B', std::regex("#{3}") },
    };
    for(auto && triomino : triominos) {
        if(triomino.exist_in(field)) {
            return triomino.name();
        }
    }
    return '-';
}

void test(const std::string & src, const char expected)
{
    const auto actual = solve(src);
    std::cout << (actual == expected ? "ok" : "***NG***") << std::endl;
}

int main()
{
    /*0*/  test( "cba", 'B' );
    /*1*/  test( "yam", '-' );
    /*2*/  test( "aaa", '-' );
    /*3*/  test( "def", '-' );
    /*4*/  test( "gga", '-' );
    /*5*/  test( "bbf", '-' );
    /*6*/  test( "gmh", 'T' );
    /*7*/  test( "mhn", 'L' );
    /*8*/  test( "dea", '-' );
    /*9*/  test( "mrn", 'R' );
    /*10*/ test( "hcm", 'I' );
    /*11*/ test( "mno", 'B' );
    /*12*/ test( "snr", 'J' );
    /*13*/ test( "xnn", '-' );
    /*14*/ test( "nnl", '-' );
    /*15*/ test( "kop", '-' );
    /*16*/ test( "ejd", 'T' );
    /*17*/ test( "txy", 'J' );
    /*18*/ test( "pvu", 'L' );
    /*19*/ test( "baf", 'R' );
    /*20*/ test( "hhc", '-' );
    /*21*/ test( "ono", '-' );
    /*22*/ test( "wxv", 'B' );
    /*23*/ test( "bdc", 'B' );
    /*24*/ test( "ojt", 'I' );
    /*25*/ test( "fkp", 'I' );
    return 0;
}