🔀merge: [Week22~23] MST 3문제 + Simulation 2문제 (#85)

* ✨feat: MST 알고리즘 풀이

* ✨feat: 시뮬레이션 알고리즘 풀이

jinny-l/week_22/BOJ_1197_최소_스패닝_트리.java

@@ -0,0 +1,81 @@
+package week_22;
+
+import java.io.BufferedReader;
+import java.io.IOException;
+import java.io.InputStreamReader;
+import java.util.Arrays;
+import java.util.Comparator;
+import java.util.StringTokenizer;
+
+public class BOJ_1197_최소_스패닝_트리 {
+
+ private static int[] parents;
+
+ public static void main(String[] args) throws IOException {
+ BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
+ StringTokenizer st = new StringTokenizer(br.readLine(), " ");
+
+ // 입력
+ int vertexCnt = Integer.parseInt(st.nextToken());
+ int edgeCnt = Integer.parseInt(st.nextToken());
+
+ Vertex[] trees = new Vertex[edgeCnt];
+ for (int i = 0; i < edgeCnt; i++) {
+ st = new StringTokenizer(br.readLine(), " ");
+ Vertex vertex = new Vertex();
+ vertex.start = Integer.parseInt(st.nextToken());
+ vertex.end = Integer.parseInt(st.nextToken());
+ vertex.cost = Integer.parseInt(st.nextToken());
+ trees[i] = vertex;
+ }
+
+ // 가중치 기준 정렬
+ Arrays.sort(trees, (Comparator.comparingLong(o -> o.cost)));
+
+ // 부모 노드 초기화
+ parents = new int[vertexCnt + 1];
+ for (int i = 0; i <= vertexCnt; i++) {
+ parents[i] = i;
+ }
+
+ // 크루스칼 알고리즘
+ int sum = 0;
+ for (int i = 0; i < edgeCnt; i++) {
+ Vertex vertex = trees[i];
+ int start = vertex.start;
+ int end = vertex.end;
+
+ if (!sameParent(start, end)) { // 사이클이 형성되지 않으면
+ union(start, end); // 부모 노드를 바꿔주고
+ sum += vertex.cost; // 가중치 더해주기
+ }
+ }
+
+ System.out.println(sum);
+ }
+
+ static class Vertex {
+ int start;
+ int end;
+ long cost;
+ }
+
+ private static void union(int a, int b) {
+ if (find(a) > find(b)) {
+ parents[find(a)] = find(b);
+ } else {
+ parents[find(b)] = find(a);
+ }
+ }
+
+ private static int find(int a) {
+ if (parents[a] == a) {
+ return a;
+ }
+ return parents[a] = find(parents[a]); // ???
+ }
+
+ private static boolean sameParent(int x, int y) {
+ return find(x) == find(y);
+ }
+}

jinny-l/week_22/BOJ_1922_네트워크_연결.java

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+package week_22;
+
+import java.io.BufferedReader;
+import java.io.IOException;
+import java.io.InputStreamReader;
+import java.util.Arrays;
+import java.util.Comparator;
+import java.util.StringTokenizer;
+
+public class BOJ_1922_네트워크_연결 {
+
+ private static int[] parents;
+
+ public static void main(String[] args) throws IOException {
+ BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
+
+ // 입력
+ int vertexCnt = Integer.parseInt(br.readLine());
+ int edgeCnt = Integer.parseInt(br.readLine());
+
+ Vertex[] trees = new Vertex[edgeCnt];
+ for (int i = 0; i < edgeCnt; i++) {
+ StringTokenizer st = new StringTokenizer(br.readLine(), " ");
+ Vertex vertex = new Vertex();
+
+ vertex.start = Integer.parseInt(st.nextToken());
+ vertex.end = Integer.parseInt(st.nextToken());
+ vertex.cost = Integer.parseInt(st.nextToken());
+
+ trees[i] = vertex;
+ }
+ // 정렬
+ Arrays.sort(trees, Comparator.comparingInt(o -> o.cost));
+
+ // 부모 노드 초기화
+ parents = new int[vertexCnt + 1];
+ for (int i = 0; i <= vertexCnt; i++) {
+ parents[i] = i;
+ }
+
+ // 크루스칼 알고리즘
+ int cost = 0;
+ for (int i = 0; i < edgeCnt; i++) {
+ Vertex vertex = trees[i];
+ int start = vertex.start;
+ int end = vertex.end;
+
+ if (!sameParent(start, end)) {
+ union(start, end);
+ cost += vertex.cost;
+ }
+ }
+ System.out.println(cost);
+ }
+
+ static class Vertex {
+ int start;
+ int end;
+ int cost;
+ }
+
+ private static boolean sameParent(int x, int y) {
+ return find(x) == find(y);
+ }
+
+ private static int find(int vertex) {
+ if (parents[vertex] == vertex) { // 여기도 왜 자기 자신일 떄는 그냥 돌아가는지 모르겠음
+ return vertex;
+ }
+ return parents[vertex] = find(parents[vertex]); // 여기 이해 못함
+ }
+
+ private static void union(int x, int y) {
+ int xParent = find(x);
+ int yParent = find(y);
+
+ if (xParent > yParent) {
+ parents[x] = yParent; // 여기 잘못됨
+ } else {
+ parents[y] = xParent;
+ }
+ }
+
+}

jinny-l/week_22/BOJ_21924_도시_건설.java

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+package week_22;
+
+import java.io.BufferedReader;
+import java.io.IOException;
+import java.io.InputStreamReader;
+import java.util.Arrays;
+import java.util.Comparator;
+import java.util.StringTokenizer;
+
+// TODO: 이해가 가지 않는 점: union find 하면서 부모 노드가 다 똑같이 변해야 하는 게 아닌가? 왜 2번 예저는 부모 노드가 다르지????
+// TODO: 이상한 입력 조건 머냐 저거,, ㅎ
+
+public class BOJ_21924_도시_건설 {
+
+ private static int[] parents;
+
+ public static void main(String[] args) throws IOException {
+ BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
+ StringTokenizer st = new StringTokenizer(br.readLine(), " ");
+
+ // 입력
+ int vertexCnt = Integer.parseInt(st.nextToken());
+ int edgeCnt = Integer.parseInt(st.nextToken());
+
+ long totalCost = 0;
+ Vertex[] trees = new Vertex[edgeCnt];
+ for (int i = 0; i < edgeCnt; i++) {
+ st = new StringTokenizer(br.readLine(), " ");
+ Vertex vertex = new Vertex();
+
+ vertex.start = Integer.parseInt(st.nextToken());
+ vertex.end = Integer.parseInt(st.nextToken());
+ vertex.cost = Integer.parseInt(st.nextToken());
+
+ trees[i] = vertex;
+ totalCost += vertex.cost;
+ }
+
+ // 가중치 기준 정렬
+ Arrays.sort(trees, (Comparator.comparingInt(o -> o.cost)));
+
+ // 부모 노드 초기화
+ parents = new int[vertexCnt + 1];
+ for (int i = 0; i <= vertexCnt; i++) {
+ parents[i] = i;
+ }
+
+ // 크루스칼 알고리즘
+ long minCost = 0;
+ int unionCnt = 0;
+ for (int i = 0; i < edgeCnt; i++) {
+ Vertex vertex = trees[i];
+ int start = vertex.start;
+ int end = vertex.end;
+
+ if (!sameParent(start, end)) {
+ union(start, end);
+ minCost += vertex.cost;
+ unionCnt++;
+ }
+ }
+ if (unionCnt != vertexCnt - 1) {
+ System.out.println(-1);
+ return;
+ }
+ System.out.println(totalCost - minCost);
+ }
+
+ static class Vertex {
+ int start;
+ int end;
+ int cost;
+ }
+
+ private static void union(int a, int b) {
+ if (find(a) > find(b)) {
+ parents[find(a)] = find(b);
+ } else {
+ parents[find(b)] = find(a);
+ }
+ }
+
+ private static int find(int a) {
+ if (parents[a] == a) {
+ return a;
+ }
+ return parents[a] = find(parents[a]); // 이런 문법 처음봄,,
+ }
+
+ private static boolean sameParent(int x, int y) {
+ return find(x) == find(y);
+ }
+}

jinny-l/week_23/BOJ_1713_후보_추천하기.java

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+package week_23;
+
+import java.io.BufferedReader;
+import java.io.IOException;
+import java.io.InputStreamReader;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.Comparator;
+import java.util.List;
+import java.util.StringTokenizer;
+
+public class BOJ_1713_후보_추천하기 {
+
+ public static void main(String[] args) throws IOException {
+ BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
+
+ // 입력
+ int postSize = Integer.parseInt(br.readLine());
+ int votes = Integer.parseInt(br.readLine());
+ StringTokenizer st = new StringTokenizer(br.readLine(), " ");
+
+ // 최종 후보 확인
+ Student[] students = new Student[101]; // 모든 학생의 정보를 담는 배열
+ List<Student> post = new ArrayList<>(); // 사진틀에 게시된 최종 후보의 학생 정보를 담는 배열
+
+ for (int i = 0; i < votes; i++) {
+ int vote = Integer.parseInt(st.nextToken());
+
+ if (students[vote] == null) { // 처음 추천받았다면 객체 생성
+ students[vote] = new Student(vote, 0, 0, false);
+ }
+
+ if (students[vote].isPosted) { // 이미 게시되어 있으면 추천수만 1증가
+ students[vote].count++; // 4번 조건 주의
+ continue;
+ }
+
+ if (post.size() < postSize) { // 사진틀이 비어있으면
+ post.add(students[vote]); // 게시하고
+
+ } else { // 사진들이 차있으면
+ Collections.sort(post); // 추천순, 시간순으로 정렬하고
+ post.get(0).isPosted = false; // 게시 여부 정보 바꾸고
+ post.get(0).count = 0; // 추천 수 0으로 변경
+ post.remove(0); // 기존 학생 제거하고
+ post.add(students[vote]); // 새로운 학생 게시
+ }
+
+ // 추천된 학생 정보 업데이트
+ students[vote].count++; // 추천수 1 증가하고
+ students[vote].time = i; // 포스팅 시간 업데이트하고
+ students[vote].isPosted = true; // 포스팅 여부 업데이트
+ }
+
+ // 출력
+ post.sort(Comparator.comparingInt(o -> o.index));
+ post.forEach(student -> System.out.print(student.index + " "));
+ }
+
+ static class Student implements Comparable<Student> {
+ int index;
+ int count;
+ int time;
+ boolean isPosted;
+
+ public Student(int index, int count, int time, boolean isPosted) {
+ this.index = index;
+ this.count = count;
+ this.time = time;
+ this.isPosted = isPosted;
+ }
+
+ @Override
+ public int compareTo(Student o) {
+ if (this.count == o.count) {
+ return this.time - o.time;
+ }
+ return this.count - o.count;
+ }
+ }
+}