200923 수 [BOJ] 2156, 11053, 11055, 11722

BOJ 2156

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

public class Main {	

    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());
    	
    	int[] glasses = new int[n+1];
    	int[] dp = new int[n+1];
    	
    	for(int i=1;i<=n;i++) {
    		glasses[i] = Integer.parseInt(br.readLine());
    	}
    	
    	if(n==1) {
    		System.out.println(glasses[1]);
    	} else if (n==2) {
    		System.out.println(glasses[1] + glasses[2]);
    	} else {
    		dp[1] = glasses[1];
    		dp[2] = dp[1] + glasses[2];
    		dp[3] = Math.max(dp[2], Math.max(glasses[1] + glasses[3], glasses[2] + glasses[3]));
    		
    		for(int i=4;i<=n;i++) {
    			dp[i] = Math.max(dp[i-3] + glasses[i] + glasses[i-1], dp[i-2] + glasses[i]);
    			dp[i] = Math.max(dp[i-1], dp[i]);
    		}
    		System.out.println(dp[n]);
    	}
    	
    	bw.flush();    	
    	bw.close();
    }
}

(출처 : https://hees-dev.tistory.com/30 )

규칙을 찾아서 일반화시키는 과정에서 길을 잃었다.. 나름 2중배열까지 사용해가며 코딩해봤으나 결국 답을 못찾고 실패. 다른 사람 코드 참고함.

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BOJ 11053

LIS(Longest Increasing Subsequence) 문제

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

public class Main {	

    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());
    	StringTokenizer st = new StringTokenizer(br.readLine());
    	br.close();
    	int[] sequence = new int[N];
    	int[] dp = new int[N];
    	for(int i=0;i<N;i++) {
    		sequence[i] = Integer.parseInt(st.nextToken());
    	}
    	
    	int max = 0;
    	for(int i=0;i<N;i++) {
    		if(dp[i]==0)
    			dp[i] = 1;
    		for(int j=0;j<i;j++) {
    			if(sequence[i]> sequence[j]) {
    				dp[i] = Math.max(dp[i], dp[j]+1);
    			}
    		}
    		if(max < dp[i])
    			max = dp[i];
    	}
    	bw.write(String.valueOf(max));
    	
    	bw.flush();    	
    	bw.close();
    }
}

(참조 : https://jins-dev.tistory.com/entry/%EC%B5%9C%EC%A0%81%ED%99%94%EB%90%9C-LISLongest-Increasing-Subsequence-%EC%95%8C%EA%B3%A0%EB%A6%AC%EC%A6%98%EA%B3%BC-%ED%95%B4-%EC%B0%BE%EA%B8%B0 )

처음 접해본 LIS 문제였다. 일단 기본 패턴을 익힐겸 다른 사람 코드를 보고 작성해봤다.
이중 for문 중 안쪽 for문에서 dp[i]값이 더 작아지지 않도록, Math.max()를 처리한 부분이 있었다.

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BOJ 11055

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

public class Main {	

    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());
    	StringTokenizer st = new StringTokenizer(br.readLine());
    	br.close();
    	int[] sequence = new int[N+1];
    	int[] dp = new int[N+1];
    	for(int i=1;i<=N;i++) {
    		sequence[i] = Integer.parseInt(st.nextToken());
    	}
    	
    	int max = 0;
    	for(int i=1;i<=N;i++) {
    		dp[i] = sequence[i];
    		for(int j=1;j<i;j++) {
    			if(sequence[j]<sequence[i] && dp[i] < dp[j] + sequence[i])
    				dp[i] = dp[j] + sequence[i];
    		}
    		if(max < dp[i])
    			max = dp[i];
    	}
    	bw.write(String.valueOf(max));
    	
    	bw.flush();    	
    	bw.close();
    }
}

LIS 응용해서 합을 구하는 문제
(참조 : https://m.blog.naver.com/occidere/220793914361 )

---

BOJ 11722

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

public class Main {	

    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());
    	StringTokenizer st = new StringTokenizer(br.readLine());
    	br.close();
    	int[] sequence = new int[N+1];
    	int[] dp = new int[N+1];
    	for(int i=1;i<=N;i++) {
    		sequence[i] = Integer.parseInt(st.nextToken());
    	}
    	int max = 0;
    	for(int i=1;i<=N;i++) {
    		if(dp[i] == 0)
    			dp[i] = 1;
    		for(int j=1;j<i;j++) {
    			if(sequence[j] > sequence[i])
    				dp[i] = Math.max(dp[i], dp[j]+1);
    		}
    		if(max < dp[i])
    			max = dp[i];
    	}
    	
    	bw.write(String.valueOf(max));
    	bw.flush();    	
    	bw.close();
    }
}

11053 문제랑 거의 유사한 문제였음. '증가' -> '감소' 차이.

주어진 배열과 인덱스를 같이하는 dp[] 를 만들어 사용하는 풀이방법이 아직도 잘 와닿지 않음.

DP(다이나믹 프로그래밍)에서는 규칙을 찾거나, 어떤 일반식을 찾는 것이 키 포인트이다.
(출처 : https://developer-mac.tistory.com/72 )