洛谷 1341 无序字母对

欧拉路的裸题？？？

#include <algorithm>
#include <iostream>
#include <cstring>
#include <cstdio>

using namespace std;

const int N = 3550;

int e[N][N], fir[4], a[N], m, tot, v[N], id[N], f[N], cnt = 0, ind[N], vec[N][N], l[N];
char s[3], sa[N];

inline bool cmp(int a, int b)
{
    return sa[a] < sa[b];
}

void DFS(int u, int ste)
{
    a[ste] = u;
    if (ste == m)
    {
        for (int i = 0; i <= m; ++i) putchar(sa[a[i]]);
        exit(0); 
    }
    for (int i = 1; i <= tot; ++i)
        if (e[u][i]) vec[u][++l[u]] = i;
    sort(vec[u] + 1, vec[u] + 1 + l[u], cmp);
    for (int i = 1; i <= l[u]; ++i)
    {
        if (!e[u][vec[u][i]]) continue;
        e[u][vec[u][i]] = e[vec[u][i]][u] = 0;
        DFS(vec[u][i], ste + 1);
        e[u][vec[u][i]] = e[vec[u][i]][u] = 1;
    }
}

int main()
{
    scanf("%d", &m);
    for (int i = 1; i <= m; ++i)
    {
        cin >> s[0] >> s[1];
        if (!v[(int)s[0]]) v[(int)s[0]] = 1, id[(int)s[0]] = ++tot, sa[tot] = s[0];
        if (!v[(int)s[1]]) v[(int)s[1]] = 1, id[(int)s[1]] = ++tot, sa[tot] = s[1];
        e[id[(int)s[0]]][id[(int)s[1]]] = e[id[(int)s[1]]][id[(int)s[0]]] = 1;
        ++ind[id[s[0]]], ++ind[id[s[1]]];
    }
    int ox = 0, ch = 0; fir[2] = 1;
    for (int i = 1; i <= tot; ++i) 
        if (ind[i] & 1) ++ox, fir[ch ^ 1] = i, ch ^= 1;
        else fir[2] = (sa[i] < (char)sa[fir[2]]) ? i : fir[2];
    if (ox == 1 || ox > 2)
    {
        puts("No Solution");
        return 0;
    }
    if (ox)    { if (sa[fir[0]] > sa[fir[1]]) swap(fir[0], fir[1]); DFS(fir[0], 0); }
    else { DFS(fir[2], 0); }
    return 0;
}