题解 | 计算某字符出现次数

import java.util.Scanner;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        // 注意 hasNext 和 hasNextLine 的区别
        while (in.hasNext()) { // 注意 while 处理多个 case
            String a = in.nextLine();
            String b = in.nextLine();
            int count = countNum(a, b);
            System.out.println(count);
        }
    }

    public static int countNum(String a, String b) {
        String regex = "^[A-Za-z0-9]+$";
        Pattern pattern = Pattern.compile(regex);
        Matcher m = pattern.matcher(b);
        if (a == null || a.isEmpty() || b == null || b.isEmpty() ||
                a.length() < b.length() || b.length() != 1 || !m.matches()) {
            return 0;
        }
        int count = 0;
        for(int i=0;i<a.length();i++){
            if((a.subSequence(i,i+1)+"").equalsIgnoreCase(b)){
                count ++;
            }
        }
        return count;
    }
}