LeetCode/cpp/ransom-note.cpp

#include <cassert>
#include <map>
#include <string>

namespace {

std::map<char, int> build_freqs(const std::string &input) {
    std::map<char, int> freqs;

    for (auto c : input) {
        freqs[c]++;
    }

    return freqs;
}

bool subtract(std::map<char, int> available, const std::string &message) {
    for (auto c : message) {
        available[c]--;

        if (available[c] < 0) {
            return false;
        }
    }

    return true;
}

} // namespace

class Solution {
  public:
    bool canConstruct(const std::string &ransomNote,
                      const std::string &magazine) {
        auto available = build_freqs(magazine);
        return subtract(available, ransomNote);
    }
};

int main() {
    Solution s;

    assert(!s.canConstruct("a", "b"));
    assert(!s.canConstruct("aa", "ab"));
    assert(s.canConstruct("aa", "aab"));
    assert(s.canConstruct("ab", "aab"));
    assert(s.canConstruct("ba", "aab"));

    return 0;
}