【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-zRandom 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-zlong s = System.currentTimeMillis();for(int i =0; i<str.length();i++){char c = str.charAt(i);  //if  c is achs[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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