codeforces 508D . Tanya and Password 欧拉通路

题目链接

给你n个长度为3的子串， 这些子串是由一个长度为n+2的串分割得来的， 求原串， 如果给出的不合法， 输出-1。

一个欧拉通路的题， 将子串的前两个字符和后两个字符看成一个点， 比如acb， 就是ac->cb。 然后建图。

 #include <iostream>
 #include <vector>
 #include <cstdio>
 #include <cstring>
 #include <algorithm>
 #include <cmath>
 #include <map>
 #include <set>
 #include <stack>
 #include <string>
 #include <queue>
 using namespace std;
 #define pb(x) push_back(x)
 #define ll long long
 #define mk(x, y) make_pair(x, y)
 #define lson l, m, rt<<1
 #define mem(a) memset(a, 0, sizeof(a))
 #define rson m+1, r, rt<<1|1
 #define mem1(a) memset(a, -1, sizeof(a))
 #define mem2(a) memset(a, 0x3f, sizeof(a))
 #define rep(i, n, a) for(int i = a; i<n; i++)
 #define fi first
 #define se second
 typedef pair<int, int> pll;
 const double PI = acos(-1.0);
 const double eps = 1e-;
 const int mod = 1e9+;
 const int inf = ;
 const int dir[][] = { {-, }, {, }, {, -}, {, } };
 map <string, int> ma;
 string a[];
 int cnt, inde[], outde[], vis[*], num, ans[*], ecnt[];
 vector <int> v[];
 void dfs(int u) {
     while(ecnt[u]<v[u].size()) {
         dfs(v[u][ecnt[u]++]);
     }
     ans[num++] = u%;
 }
 int main()
 {
     ios::sync_with_stdio();
     string s, tmp;
     int n, pos1, pos2;
     cin>>n;
     int cnt = , start;
     for(int i = ; i<n; i++) {
         cin>>s;
         int u = *s[]+s[];
         int to = *s[]+s[];
         v[u].pb(to);
         inde[to]++;
         outde[u]++;
         start = u;
     }
     pos1 = -, pos2 = -;
     int flag = ;
     for(int i = ; i<; i++) {
         if(outde[i]!=inde[i]) {
             if(outde[i]==inde[i]+) {
                 if(pos1==-) {
                     pos1 = i;
                 }  else {
                     flag = ;
                 }
             } else {
                 if(pos2==-)
                     pos2=i;
                 else
                     flag = ;
             }
         }
     }
     if(flag||(pos1==-&&pos2!=-||pos2==-&&pos1!=-)) {
         cout<<"NO"<<endl;
         return ;
     }
     num = ;
     if(pos1 == -)
         pos1 = start;
     dfs(pos1);
     if(num != n+) {
         puts("NO");
         return ;
     }
     s = "", tmp = "";
     tmp = (char)(pos1/);
     for(int i = num-; i>=; i--) {
         tmp += char(ans[i]);
     }
     cout<<"YES"<<endl;
     cout<<tmp;
     return ;
 }