【每日一练】20231023

统计每个字符出现的次数相关问题

方法一：map的put方法+遍历

public class Test {

    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder("");
        Random ran = new Random();
        for(int i=0;i<2000000;i++) {
            sb.append((char) ('a' + ran.nextInt(27)));
        }
        String str = sb.toString();
        //System.out.println(sb);
        

        //统计一下每个字符出现的次数？出现最多？
        Map<Character, Integer> map = new HashMap<>();
        long s = System.currentTimeMillis();
        for (int i = 0; i < str.length(); i++) {
            Character ch = str.charAt(i);
            if (Objects.isNull(map.get(ch))) {
                map.put(ch, 1);
            } else {
                map.put(ch, map.get(ch) + 1);
            }
        }
        long e = System.currentTimeMillis();
        System.out.println((e-s)+"ms");

        // System.out.println(map);

    }
}

方法二：用数组

public class Test2 {

    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder("");  //a-z
        Random ran = new Random();
        for(int i=0;i<2000000;i++) {
            sb.append((char) ('a' + ran.nextInt(26)));
        }
        String str = sb.toString();
//        System.out.println(str);
        int[] chs =new int[26];  //chs[0] chs[25]  0-a, 25-z

        long s = System.currentTimeMillis();
        for(int i =0; i<str.length();i++){
            char c = str.charAt(i);  //if  c is a
            chs[c-'a']+=1;

        }
        long e = System.currentTimeMillis();
        System.out.println((e-s)+"ms");
//
//        for(int i=0;i<chs.length;i++){
//            System.out.print((char)(i+'a')+",");
//            System.out.println(chs[i]);
//        }
        
    }
}

方法三：map的getOrDefault方法

public class Test3 {
    public static void main(String[] args) {
        String str = "The so-called 'multifunctional' is not adding many elements, " +
                "but subtracting them. Only by leaving blank can ten thousand possibilities be created.";

        // 创建HashMap用于存储字母和出现次数
        Map<Character, Integer> countMap = new HashMap<>();

        // 遍历字符串的每个字符
        for (char c : str.toCharArray()) {
            // 判断是否为字母
            if (Character.isLetter(c)) {
                // 将字母转为小写形式
                char lowercase = Character.toLowerCase(c);

                // 更新字母的出现次数，使用Map的getOrDefault方法来获取该字母已有的计数并加一
                countMap.put(lowercase, countMap.getOrDefault(lowercase, 0) + 1);
            }
        }
        // 遍历Map输出每个字母及其出现次数
        for (char c : countMap.keySet()) {
            int count = countMap.get(c);
            System.out.println(c + ": " + count);
        }
    }
    
}

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