Following order Indegree, C++ (gcc)

Language: Layout:

#include<bits/stdc++.h>
using namespace std;
#define ll long long int
const int N=1e6;
vector<int> v[23];
int a[23];
vector<string> ans[23];
vector<int> inc(23);
int in[23];
int m[23];
int main()
{
    char c;
    int n=0;
    while
    while(1)
    {
        cin>>c;
        if(c=='/n') break;
        else        m[c-'a']=n,a[n++]=c-'a';
    }
    char c1,c2;
    while(1)
    {
        cin>>c1>>c2;
        if(c1=='/n') break;
        else         v[m[c1-'a']].push_back(m[c2-'a']),in[m[c2-'a']]++;
    }
    for(int i=0;i<n;i++)
    {
        inc[in[i]].push_back(i);
    }
    for(auto it : inc[0])
    {
        ans[it].push_back((string)(a[it]+'a'));
    }
    while(1)
    {
        if(inc[0].size()==0)
        {
            break;
        }
        for(auto it : inc[0])
        {
            for(auto des : v[it])
            {
                for(auto out : ans[it])
                {
                    ans[des].push_back(out+(string)(a[des]+'a'+""));
                }
                inc(in[des]).erase(des);
                inc(in[des]-1).push_back(des);
            }
            inc[0].erase(it);
        }
    }
}

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Compilation time: 1.52 sec, absolute running time: 0.11 sec, cpu time: 0.07 sec, memory peak: 47 Mb, absolute service time: 1,64 sec