How to find the longest alphebetical substring c++

Question

I am trying to write a function that when given a string, it will return the longest substring that is sorted alphabetically. This has proven very difficult and despite many attempts I am no closer than when I began.

An example of what the function should do:

abacdefkabfhxy should return abcdefkxy abacdefkabfhixy should return abcdefhixy

Thanks for any help!

Answer 1

Try the following. It doesn't check if the characters are alphabetic, but you can easily add that condition yourself:

#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 = "ghijkdefghijabcde";
    std::cout << longest_alpha_substr(str); // "defghij"
}

Answer 2

For each alphabet give values as a=1,b=2...z=26.

Now solve Longest Increasing sub sequence problem.

You will get a sequence of increasing numbers.

Convert them back to alphabets and you are done.

A[1..n] - input sequence L[j] = longest strictly increasing subsequence ending at position j

Recurrence eqn:

L[j] = max of i such that i<j & A[i] <A[j] {L[i]} + 1