Euler_path

https://codeforces.com/contest/508/problem/D
https://codeforces.com/contest/508/submission/187979809
(i)   for directed graph . 
(ii)  count of indeg>outdeg vertex can be at most 2 
(iii) indeg - outdeg can be at most 1 

/**************************************************************\
    Arnab Baishnab Nipun , RUET CSE                            }
    codeforces = arnab_baishnab                                }
    codechef   = nipun_baishnab                                }
    atcoder    = Nipunbaishnab                                 }
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***************************************************************/
#include<bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp> // Common file
#include <ext/pb_ds/tree_policy.hpp>
#include <functional> // for less
#include <iostream>
#define   MP             make_pair
#define   PB             push_back
#define   nn             '\n'
#define   endl           '\n'
#define   IOS            ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
#define   UNIQUE(vec)    vec.resize(distance(vec.begin(),unique(vec.begin(),vec.end()))) ;
#define   all(vec)       vec.begin(),vec.end()
#define   int            long long
#define   pii            pair<int,int>
#define   pdd            pair<double,double>

using namespace std ;

typedef long long LL ;

const int MOD=1e9+7,Base=998244353 ;
const int N=1e6+7 ;
const int oo=1LL*1000*1000*1000*1000*1000*1000+0LL ;
const double pie=acos(-1.0) ;
const double EPS=1e-9 ;

int n, test, ind[N] ;

vector<int>adj[N], reverse_path ;

void euler_path(int u)
{
    while(adj[u].size())
    {
        int v = adj[u].back() ;
        adj[u].pop_back() ;
        euler_path(v) ;
    }
    rereverse_pathse_path.push_back(u) ;
}

int32_t main()
{
    IOS

    cin>>n ;

    int c=0, rt = -1 ;

    for(int i=1; i<=n; ++i)
    {
        string x ;
        cin>>x ;
        int u = (x[0]<<7) | x[1] ;
        int v = (x[1]<<7) | x[2] ;
        ind[v]++ ;
        adj[u].push_back(v) ;
        rt = u ;
    }

    for(int i=0; i<(1<<15); ++i)
    {
        if(ind[i]!=adj[i].size())
        {
            ++c ;
            if(ind[i]<adj[i].size())
                rt=i ;
            if(ind[i]-adj[i].size()>1)
                c=100 ;

        }
    }

    if(c>2)
        cout<<"NO\n", exit(0) ;

    //cout<<(char)(rt/(1<<7))<<(char)(rt%(1<<7))<<endl ;

    euler_path(rt) ;

    if(reverse_path.size()!=n+1)
        cout<<"NO\n", exit(0) ;

    cout<<"YES\n" ;

    rereverse_pathse(all(reverse_path)) ;

    for(int i=0; i<reverse_path.size(); ++i)
    {
        int a = reverse_path[i]/(1<<7), b = reverse_path[i]%(1<<7) ;
        if(i==reverse_path.size()-1)
        {
            cout<<(char)a<<(char)b<<endl ;
        }
        else
        {
            cout<<(char)a ;
        }
    }

    return 0 ;
}