poj 1386 Play on Words（有向图欧拉路+并查集）

#include <cstdio>
#include <cstring>
#include <iostream>
using namespace std;

const int MAX_N = 30;
const int MAX_M = 1000 + 10;
int fa[MAX_N], show[MAX_N];
int in[MAX_N], out[MAX_N];
int set_visited[MAX_N];
char str[MAX_M];

void Init()
{
    for (int i = 0; i < MAX_N; ++i)
        fa[i] = i;
    memset(in, 0, sizeof(in));
    memset(out, 0, sizeof(out));
    memset(show, 0, sizeof(show));
    memset(set_visited, 0, sizeof(set_visited));
}

int Find(int a)
{
    if (fa[a] == a)
        return a;
    else
        return fa[a] = Find(fa[a]);
}

int Union(int a, int b)
{
    int fa_a = Find(a);
    int fa_b = Find(b);

    if (fa_a == fa_b)
        return -1;
    if (fa_a > fa_b)
        fa[fa_b] = fa_a;
    else
        fa[fa_a] = fa_b;
    return 1;
}

int main()
{
    int case_times, n, len;

    scanf("%d", &case_times);
    while (case_times--)
    {
        scanf("%d", &n);
        Init();
        for (int i = 0; i < n; ++i)
        {
            int l, r;

            scanf("%s", str);
            len = strlen(str);
            l = str[0] - ‘a‘;
            r = str[len - 1] - ‘a‘;
            in[l]++;
            out[r]++;
            show[l] = show[r] = 1;
            Union(l, r);
        }
        int set_count = 0;
        int not_equal = 0;
        int in_big_out = 0, out_big_in = 0;
        bool ok = true;
        for (int i = 0; i < MAX_N; ++i)
        {
            if (show[i])
            {
                int fa_i = Find(i);
                if (set_visited[fa_i] == 0)
                {
                    set_count++;
                    set_visited[fa_i] = 1;
                }
                if (set_count > 1)
                {
                    ok = false;
                    break;
                }
                if (in[i] != out[i])
                    not_equal++;
                if (in[i] == out[i] + 1)
                    in_big_out++;
                if (in[i] + 1 == out[i])
                    out_big_in++;
            }
        }

        if (!((not_equal == 0) || (out_big_in == 1 && in_big_out == 1
            && in_big_out + out_big_in == not_equal)))
            ok = false;

        if (ok)
            printf("Ordering is possible.\n");
        else
            printf("The door cannot be opened.\n");
    }
    return 0;
}