poj 1386 有向图欧拉（回）路

判断是否存在欧拉（回）路，注意先要用并查集判断图是否连通。

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

const int N = 26;
const int M = 1001;
int f[N];
int degree[N];
bool visit[N];
char word[M];

int findf( int x )
{
    if ( f[x] != x ) f[x] = findf(f[x]);
    return f[x];
}

void union_set( int x, int y )
{
    x = findf(x), y = findf(y);
    if ( x != y )
    {
        f[x] = y;
    }
}

bool judge() 
{
    int cnt = 0;
    for ( int i = 0; i < N; i++ )
    {
        if ( visit[i] && f[i] == i ) cnt++;
    }
    if ( cnt > 1 ) return false;
    int p = 0, q = 0;
    for ( int i = 0; i < N; i++ )
    {
        if ( degree[i] == 1 )
        {
            p++;
        }
        else if ( degree[i] == -1 )
        {
            q++;
        }
        else if ( degree[i] )
        {
            return false;
        }
    }
    return p <= 1 && p == q;
}

int main ()
{
    int t;
    scanf("%d", &t);
    while ( t-- )
    {
        int n;
        scanf("%d", &n);
        for ( int i = 0; i < N; i++ ) f[i] = i;
        memset( degree, 0, sizeof(degree) );
        memset( visit, 0, sizeof(visit) );
        for ( int i = 0; i < n; i++ )
        {
            scanf("%s", word);
            int first = word[0] - 'a', last = word[strlen(word) - 1] - 'a';
            degree[first]--;
            degree[last]++;
            visit[first] = visit[last] = true;
            union_set( first, last );        
        }
        if ( judge() ) printf("Ordering is possible.
");
        else printf("The door cannot be opened.
");
    }
    return 0;
}