(BOJ)蓄積エネルギー161998回

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1.優先キュー

コード#コード#

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.util.PriorityQueue;

public class Main {

  static boolean[] isSelected;
  static int[] arr; // 값
  static int[] left; // 유효한 왼쪽 인덱스
  static int[] right; // 유효한 오른쪽 인덱스

  static class Node implements Comparable<Node>{
    int m;
    int l;
    int r;

    Node (int m, int l, int r) {
      this.m = m;
      this.l = l;
      this.r = r;
    }

    @Override
    public int compareTo(Node o) {
      return arr[o.l] * arr[o.r] - arr[this.l] * arr[this.r];
    }
  }

  public static void main(String[] args) throws IOException {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(System.out));

    int n = Integer.parseInt(br.readLine());
    String[] inputs = br.readLine().split(" ");

    PriorityQueue<Node> pq = new PriorityQueue<>();
    arr = new int[n];
    left = new int[n];
    right = new int[n];

    for (int i = 0; i < n; i++) {
      arr[i] = Integer.parseInt(inputs[i]);
    }

    for (int i = 1; i < n - 1; i++) {
      pq.add(new Node(i, i - 1, i + 1));
      left[i] = i - 1;
      right[i] = i + 1;
    }

    int sum = 0;

    while (!pq.isEmpty()) {
      Node node = pq.remove();

      int lPt = node.l;
      int rPt = node.r;
      int mPt = node.m;

      // pq에 넣어둔 본인 기준 좌우가 현재 상태에서 본인 기준 좌우가 다르면 건너뛴다. 
      // (기존에 넣어둔 좌우 중 누군가는 사라졌다는 의미)
      if (left[mPt] != lPt || right[mPt] != rPt) {
        continue;
      }

      sum += arr[lPt] * arr[rPt];

      if (lPt != 0) {
        right[lPt] = right[mPt];
        pq.add(new Node(lPt, left[lPt],  right[lPt]));
      }

      if (rPt != n - 1) {
        left[rPt] = left[mPt];
        pq.add(new Node(rPt, left[rPt],  right[rPt]));
      }
    }

    bw.write(Integer.toString(sum));
    bw.flush();
    bw.close();
  }
}

反例

DFS

コード#コード#

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.util.PriorityQueue;

public class Main {

  static boolean[] isSelected;
  static int[] arr; // 값
  static int[] left; // 유효한 왼쪽 인덱스
  static int[] right; // 유효한 오른쪽 인덱스
  static boolean[] isDead;
  static int n;
  static int answer = 0;

  public static void main(String[] args) throws IOException {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(System.out));

    n = Integer.parseInt(br.readLine());
    String[] inputs = br.readLine().split(" ");

    arr = new int[n];
    left = new int[n];
    right = new int[n];
    isDead = new boolean[n];

    for (int i = 0; i < n; i++) {
      arr[i] = Integer.parseInt(inputs[i]);
    }

    for (int i = 1; i < n - 1; i++) {
      left[i] = i - 1;
      right[i] = i + 1;
    }

    dfs(0, 0);

    bw.write(Integer.toString(answer));

    bw.flush();
    bw.close();
  }

  static void dfs(int depth, int sum) {
    if (depth == n - 2) {
      if (sum > answer) {
        answer = sum;
      }
      return;
    }

    for (int i = 1; i < n - 1; i++) {
      if (!isDead[i]) {
        isDead[i] = true;
        int l = left[i];
        int r = right[i];
        int value = arr[l] * arr[r];

        if (l != 0) {
          right[l] = right[i];
        }

        if (r != n - 1) {
          left[r] = left[i];
        }

        // 호출
        dfs(depth + 1, sum + value);

        right[l] = i;
        left[r] = i;
        isDead[i] = false;
      }
    }
  }
}