某省调查城镇交通状况，得到现有城镇道路统计表，表中列出了每条道路直接连通的城镇。省政府“畅通工程”的目标是使全省任何两个城镇间都可以实现交通（但不一定有直接的道路相连，只要互相间接通过道路可达即可）。问最少还需要建设多少条道路？
测试输入包含若干测试用例。每个测试用例的第1行给出两个正整数，分别是城镇数目N ( < 1000 )和道路数目M；随后的M行对应M条道路，每行给出一对正整数，分别是该条道路直接连通的两个城镇的编号。为简单起见，城镇从1到N编号。
注意:两个城市之间可以有多条道路相通,也就是说
3 3
1 2
1 2
2 1
这种输入也是合法的
当N为0时，输入结束，该用例不被处理。
对每个测试用例，在1行里输出最少还需要建设的道路数目。
4 2
1 3
4 3
3 3
1 2
1 3
2 3
5 2
1 2
3 5
999 0
0
output
1
0
2
998
Huge input, scanf is recommended.
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import java.io.*;
import java.util.*;
public class Main{
public static int pre[];
public static int m,n;
public static Edge []edges;
static class Edge implements Comparable<Edge>{
public int u,v,w;
public int compareTo(Edge e){
return this.w-e.w;
}
}
public static int find(int x){
int r = x;
while(pre[r]!=r) r=pre[r];
int j=x,tmp;
while(j!=r)
{
tmp = pre[j];
pre[j] = r;
j = tmp;
}
return r;
}
public static int kruskal(){
int tot=1,sum=0;
for(int i=1;i<=m&&tot<=n;i++)
{
int a = find(edges[i].u);
int b = find(edges[i].v);
if(a!=b){
sum+=edges[i].w;
pre[a] = b;
tot++;
}
}
return sum;
}
static class InputReader {
public BufferedReader reader;
public StringTokenizer tokenizer;
public InputReader(InputStream stream) {
reader = new BufferedReader(new InputStreamReader(stream), 32768);
tokenizer = null;
}
public String next() {
while (tokenizer == null || !tokenizer.hasMoreTokens()) {
try {
tokenizer = new StringTokenizer(reader.readLine());
} catch (IOException e) {
throw new RuntimeException(e);
}
}
return tokenizer.nextToken();
}
public int nextInt() {
return Integer.parseInt(next());
}
}
public static void main(String []args) {
InputReader sc = new InputReader(System.in);
while(true){
n = sc.nextInt();
m = n*(n-1)/2;
if(n==0) break;
pre = new int[n+1];
for(int i=1;i<=n;i++)
pre[i] = i;
edges = new Edge[m+1];
for(int i=1;i<=m;i++)
edges[i] = new Edge();
int c;
for(int i=1;i<=m;i++)
{
edges[i].u = sc.nextInt();
edges[i].v = sc.nextInt();
edges[i].w = sc.nextInt();
c = sc.nextInt();
if(c==1) edges[i].w = 0;
}
Arrays.sort(edges,1,edges.length);
System.out.println(kruskal());
}
}
}
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import java.util.Scanner;
public class Demo1 {
static class Union{
private int parent[];
private int count;
public Union(int count){
parent = new int[count+1];
this.count = count;
for (int i = 1; i <= count; i++) {
parent[i] = i;
}
}
int find(int p){
return p == parent[p] ? p : find(parent[p]);
}
void unionElements(int p,int q){
int pRoot = find(p);
int qRoot = find(q);
if (pRoot==qRoot){
return;
}
parent[pRoot] = qRoot;
}
}
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
int count = 0;
while (sc.hasNext()){
count = 0;
int n = sc.nextInt();
if (n==0) break;
int m = sc.nextInt();
Union union = new Union(n);
for (int i = 0; i < m; i++) {
int x = sc.nextInt();
int y = sc.nextInt();
union.unionElements(x,y);
}
for (int i = 1; i <= n; i++) {
if (union.parent[i]==i){
count++;
}
}
System.out.println(count-1);
}
}
}
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参考文章