HackerRank2

pair.cpp

#include <iostream>
#include <string>
#include <cctype>
#include <cassert>

namespace detail {
    bool isMatchingPair(char c1, char c2) {
        return std::isupper(c1) && std::tolower(c1) == c2;
    }
}

int findMatchingPair(const std::string& input) {
    using detail::isMatchingPair;
     
    if (!input.size() || !std::isupper(input[0])) {
        return -1;
    }
    
    char lastUpper = input[0];
    char const* lastLower = nullptr;
    char prev = input[0];
    int idx = -1;
    
    for (int i = 1; i < input.size(); ++i) {
        if (isMatchingPair(lastUpper, input[i])) {
            lastLower = &input[i];
            idx = i;
        }
        else if (std::isupper(input[i])) {
            prev = input[i];
        }
        else if (lastLower && *lastLower == input[i]) {
            return idx;
        }
        else if (isMatchingPair(prev, input[i])) {
            lastLower = &input[i];
            idx = i;
        } else {
            return idx;
        }
    }
    
    return idx;
}

int main() {
    assert(findMatchingPair("ABba") == 3);
    assert(findMatchingPair("ABbCca") == 5);
    assert(findMatchingPair("ABbba") == 2);
}