统计字母出现频率

要求：输出某个英文文本文件中 26 字母出现的频率，由高到低排列，并显示字母出现的百分比，精确到小数点后面两位。

代码：

import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.IOException;
import java.util.Scanner;

public class tongJi {
    public static void main(String[] args) throws IOException {
        File file = new File("D:\Harry Potter\harry potter\Harry Potter and the Sorcerer's Stone.txt");
        if (!file.exists()) {
            System.out.println("文件不存在");
            return;
        }
        BufferedReader buf = new BufferedReader(new FileReader(file));

        int[] num = new int[100];
        char[] ying = new char[100];
        String s = buf.readLine();
        while (s != null) {
            if (s.equals("exit")) {
                System.out.println("文件存在");
                System.exit(1);
            } else {
                String[] lineWords = s.split(" ");
                for (int i = 0; i < lineWords.length; i++) {
                    for (int j = 0; j < lineWords[i].length(); j++) {
                        if (lineWords[i].charAt(j) >= 'A' && lineWords[i].charAt(j) <= 'Z')
                            num[lineWords[i].charAt(j) - 'A' + 1]++;
                        else if (lineWords[i].charAt(j) >= 'a' && lineWords[i].charAt(j) <= 'z')
                            num[lineWords[i].charAt(j) - 'a' + 1 + 24]++;
                    }
                }
                s = buf.readLine();
                
            }
        }
        char a = 'A';
        char b = 'a';
        for (int i = 1; i <= 52; i++) {
            if (i <= 26)
                ying[i] = a++;
            else
                ying[i] = b++;
        }

        
        int sum = 0;
        for (int i = 1; i <= 52; i++) {
            sum += num[i];
        }
        
        for (int i = 1; i <= 52; i++) {
            for (int j = i + 1; j <= 52; j++) {
                if (num[i] < num[j]) {
                    int t = num[i];
                    num[i] = num[j];
                    num[j] = t;

                    char m = ying[i];
                    ying[i] = ying[j];
                    ying[j] = m;
                }
            }
        }
        System.out.println("字母总数："+sum);
        System.out.println("按频率从大到小排列：");
        for (int i = 1; i <= 52; i++) {
            double ans = num[i] * 1.0 / sum * 100;
            System.out.println(ying[i] + " frequency is " + String.format("%.2f", ans) + "%");
            
        }
    }
}