LA 3644 并查集判断无向图是否有环

理解清楚题意以后就明白为什么要用并查集了。

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

const int N = 100001;
int f[N];
int ans;

void init()
{
    ans = 0;
    for ( int i = 0; i < N; i++ )
    {
        f[i] = i;
    }
}

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

bool union_set( int x, int y )
{
    x = findf(x), y = findf(y);
    if ( x == y ) return false;
    f[x] = y;
    return true;
}

int main ()
{
    int x, y;
    while ( scanf("%d%d", &x, &y) != EOF )
    {
        init();
        f[x] = y;
        while ( scanf("%d", &x), x != -1 )
        {
            scanf("%d", &y);
            if ( !union_set( x, y ) )
            {
                ans++;
            }
        }
        printf("%d
", ans);
    }
    return 0;
}