1 void euler(int u) {
2     for(int v=1;v<=n;++v)
3         if(G[u][v]) {
4             G[u][v]=G[v][u]=0; //有向图则改为G[u][v]=0;
5             euler(v);
6         }
7     ans.push(u);
8 }

 1 #include <cstdio>
 2 #include <stack>
 3
 4 using namespace std;
 5
 6 const int maxa = 55;
 7
 8 inline int toInt(char c) {
 9     if (c <= 'Z') return c - 'A' + 1;
10     else return c - 'a' + 27;
11 }
12
13 inline char toChar(int i) {
14     if (i <= 26) return i - 1 + 'A';
15     else return i - 27 + 'a';
16 }
17
18 int G[maxa][maxa], degree[maxa];
19
20 stack<int> ans;
21
22 void euler(int u) {
23     for (int v = 1; v <= 52; ++v)
24         if (G[u][v]) {
25             G[u][v] = G[v][u] = 0;
26             euler(v);
27         }
28     ans.push(u);
29 }
30
31 int main() {
32     int n, a, b, s1 = 0, s2 = 0, cnt = 0;
33     scanf("%d", &n);
34     for (int i = 1; i <= n; ++i) {
35         char u = getchar();
36         while (u == '\n' || u == ' ' || u == '\r')
37             u = getchar();
38         char v = getchar();
39         a = toInt(u), b = toInt(v);
40         G[a][b] = G[b][a] = 1;
41         ++degree[a], ++degree[b];
42     }
43     for (int i = 1; i <= 52; ++i) {
44         if (degree[i] && !s1) s1 = i;
45         if (degree[i] % 2) {
46             ++cnt;
47             if (!s2) s2 = i;
48         }
49     }
50     if (cnt && cnt != 2) printf("No Solution");
51     else {
52         if (cnt) euler(s2);
53         else euler(s1);
54         if ((int)ans.size() != n + 1) printf("No Solution");
55         else while (!ans.empty()) {
56             printf("%c", toChar(ans.top()));
57             ans.pop();
58         }
59     }
60     return 0;
61 }