题解 | DNA序列

import java.util.Scanner;
import java.util.*;
// 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();
            int b = in.nextInt();
            System.out.println(findFirstDnaStr(a,b));
        }
    }

    public static String findFirstDnaStr(String str, int num) {
        if (str.length() <= num) {
            return str;
        }
        Map<Double, String> map = new HashMap<>();
        for (int i = 0; i < str.length() - num; i++) {
            String subStr = str.substring(i, i + num);
            double gc = 0;
            for (char c : subStr.toCharArray()) {
                if (c == 'G' || c == 'C') {
                    gc++;
                }
            }
            double gcRatio = gc / num;
            if (!map.containsKey(gcRatio)) {
                map.put(gcRatio, subStr);
            }
        }
        return map.get(map.keySet().stream().max(Double::compare).get());
    }
}