A function that finds the largest lexographically-increasing substring in a string.

gistfile1.cpp

#include <iostream>
#include <vector>
#include <utility>
#include <algorithm>
#include <string>

std::string longest_alpha_substr(const std::string& str)
{
    char last = str[0];
    int size = 1;
    unsigned int i = 1;
    std::vector<std::pair<int, int>> pairs;
    
    for (; i < str.size(); ++i)
    {
        if (str[i] >= last)
        {
            last = str[i];
            ++size;
        } else
        {
            pairs.push_back(std::make_pair(i - size, size));
            size = 1;
            last = str[i];
        }
    }
    
    pairs.push_back(std::make_pair(i - size, size));
    
    using pair_type = std::pair<int, int>;
    
    auto max = std::max_element(pairs.begin(), pairs.end(), [] (const pair_type& p1, const pair_type& p2)
    {
        return p1.second < p2.second;
    });
    
    return str.substr(max->first, max->second);
}

int main()
{
    std::string str = "ghijklmnopqdefghijabcdefghi";
    std::cout << longest_alpha_substr(str);
}