计算阶乘中的位数-输入/输出性能问题 [英] Count number of digits in factorial - Input/Ouput performance issue

问题描述

我正在spoj平台上解决任务-计算阶乘中的位数.我找到了Kamenetsky公式并实现了它:

I am solving task on spoj platform - count number of digits in factorial. I found Kamenetsky Formula and implemented it:

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

public class Main {
    public static void main(String[] args) throws IOException {
        BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
        int numOfTests = Integer.parseInt(in.readLine());

        for(int i = 0; i < numOfTests; i++) {
            System.out.println(KamenetskyFormula(Integer.parseInt(in.readLine())));
        }

        /*
        in.lines()
                .limit(numOfTests)
                .map(n -> Integer.parseInt(n))
                .forEach(n -> System.out.println(KamenetskyFormula(n)));*/
    }

    private static long KamenetskyFormula(int n) {
        if (n < 2) {
            return 1;
        }
        double x = n * Math.log10(n / Math.E) + Math.log10(2 * Math.PI * n) / 2.0;
        return (long) (Math.floor(x) + 1);
    }
}

首先，我使用注释的代码(流)，因为我认为它比实际的代码(没有注释)要慢，所以我进行了更改，但仍然超出了时间限制.我怎样才能让它更快?

first I used commented code (streams) and as I thought it's slower than actual one (without comments) so I changed but still getting exceed time limit. How can I make it faster?

示例输入为(第一行是测试数量):

Example input is (first line is number of tests):

3
1
10
100

和预期输出:

1
7
158

推荐答案

我认为由于I/O缓慢而超出了时间限制.这主要是因为System.out.println的基础PrintStream.在这篇文章中找到更多详细信息 why-is-system-out-println-so-慢.您可以参考下面的快速I/O模板，这将有助于解决此问题.

I think the time limit is exceeding because of the slow I/O. This is mainly because of System.out.println's underlying PrintStream. Find more details in this post why-is-system-out-println-so-slow. You can refer to the below Fast I/O template which will help solve this problem.

参考- Java中的快速I/O

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.util.InputMismatchException;

public class YourClassName {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        TaskA solver = new TaskA();
        int testCount = Integer.parseInt(in.next());
        for (int i = 1; i <= testCount; i++)
            solver.solve(i, in, out);
        out.close();
    }

    static class TaskA {
        public void solve(int testNumber, InputReader sc, PrintWriter w) {
            /*your logic goes here
            use w.println here to print the output which is usually faster
            */
        }

    }

    static class InputReader {
        private InputStream stream;
        private byte[] buf = new byte[1024];
        private int curChar;
        private int numChars;
        private InputReader.SpaceCharFilter filter;

        public InputReader(InputStream stream) {
            this.stream = stream;
        }

        public int read() {
            if (numChars == -1)
                throw new InputMismatchException();

            if (curChar >= numChars) {
                curChar = 0;
                try {
                    numChars = stream.read(buf);
                } catch (IOException e) {
                    throw new InputMismatchException();
                }

                if (numChars <= 0)
                    return -1;
            }
            return buf[curChar++];
        }

        public int nextInt() {
            int c = read();

            while (isSpaceChar(c))
                c = read();

            int sgn = 1;

            if (c == '-') {
                sgn = -1;
                c = read();
            }

            int res = 0;
            do {
                if (c < '0' || c > '9')
                    throw new InputMismatchException();
                res *= 10;
                res += c - '0';
                c = read();
            } while (!isSpaceChar(c));

            return res * sgn;
        }

        public long nextLong() {
            int c = read();
            while (isSpaceChar(c))
                c = read();
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = read();
            }
            long res = 0;

            do {
                if (c < '0' || c > '9')
                    throw new InputMismatchException();
                res *= 10;
                res += c - '0';
                c = read();
            } while (!isSpaceChar(c));
            return res * sgn;
        }

        public String readString() {
            int c = read();
            while (isSpaceChar(c))
                c = read();
            StringBuilder res = new StringBuilder();
            do {
                res.appendCodePoint(c);
                c = read();
            } while (!isSpaceChar(c));

            return res.toString();
        }

        public boolean isSpaceChar(int c) {
            if (filter != null)
                return filter.isSpaceChar(c);
            return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }

        public String next() {
            return readString();
        }

        public interface SpaceCharFilter {
            public boolean isSpaceChar(int ch);

        }

    }
}

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