CodeForces Roads not only in Berland(并查集)

H - Roads not only in Berland

Time Limit:2000MS Memory Limit:262144KB 64bit IO Format:%I64d
& %I64u

Description

Berland Government decided to improve relations with neighboring countries. First of all, it was decided to build new roads so that from each city of Berland and neighboring countries it became possible to reach all the others. There are n cities
in Berland and neighboring countries in total and exactly n - 1 two-way roads. Because of the recent financial crisis, the Berland Government is strongly pressed for money, so to build a new
road it has to close some of the existing ones. Every day it is possible to close one existing road and immediately build a new one. Your task is to determine how many days would be needed to rebuild roads so that from each city it became possible to reach
all the others, and to draw a plan of closure of old roads and building of new ones.

Input

The first line contains integer n (2 ≤ n ≤ 1000) — amount of cities in Berland and neighboring countries. Next n - 1 lines
contain the description of roads. Each road is described by two space-separated integers a_i, b_i (1 ≤ a_i, b_i ≤ n, a_i ≠ b_i)
— pair of cities, which the road connects. It can't be more than one road between a pair of cities. No road connects the city with itself.

Output

Output the answer, number t — what is the least amount of days needed to rebuild roads so that from each city it became possible to reach all the others. Then output t lines
— the plan of closure of old roads and building of new ones. Each line should describe one day in the format i j u v — it means that road between cities i and j became
closed and a new road between cities u and v is built. Cities are numbered from 1. If the answer is not unique, output any.

Sample Input

Input

2

1 2

Output

Input

Output

1

3 1 3 7

这道题目是简单的并查集应用，直接给出AC代码
#include <iostream>

#include <string.h>

#include <stdlib.h>

#include <algorithm>

#include <math.h>

#include <stdio.h>

using namespace std;

int father[1005];

int find(int x)

{

	if(x!=father[x])

		father[x]=find(father[x]);

	return father[x];

}

struct Node

{

	int x,y;

}c[1005];

int num;

int tag[1005];

int res[1005];

int main()

{

	int n;

	int a,b;

	scanf("%d",&n);

	for(int i=1;i<=1000;i++)

		father[i]=i;

	int cnt=0;int num=0;

	for(int i=1;i<=n-1;i++)

	{

         scanf("%d%d",&a,&b);

		 int fa=find(a);

		 int fb=find(b);

		 if(fa!=fb)

		 {

			 father[fa]=fb;

		 }

		 else

		 {

             c[cnt].x=a;

			 c[cnt++].y=b;

		 }

	}

	memset(tag,0,sizeof(tag));

	int cot=0;

	for(int i=1;i<=n;i++)

	{

		if(!tag[find(i)])

		{

			res[cot++]=find(i);

			num++;

			tag[find(i)]=1;

		}

	}

	printf("%d\n",num-1);

	int tot=1;

	for(int i=0;i<num-1;i++)

	{

         printf("%d %d",c[i].x,c[i].y);

		 printf(" %d %d\n",res[0],res[tot]);

		 tot++;

	}

	return 0;

}

巴特西

CodeForces Roads not only in Berland(并查集)

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