学习笔记

十进制转二进制 十进制转换成二进制 Integer.toBinaryString(num);

package list;

import java.util.ArrayList; import java.util.List;

public class mylist { public static void main(String[] args) { List list = new ArrayList<>();

    list.add("a");
    list.add("b");
    list.add("c");
    list.add("d");
    list.add("e");
    list.add(5,"f");
    list.set(0,"qqq");
    System.out.println(list);
    System.out.println(list.get(5));
}

}

//题目：有1、2、3、4个数字，能组成多少个互不相同且无重复数字的三位数？都是多少？

public class jva1209 { public static void main(String[] args) { //int [] a = {1,2,3,4}; for (int i = 1; i < 6; i++) { for (int j = 1; j < 5; j++) { for (int k = 1; k < 5; k++) { if(i != j && i != k && j != k){ System.out.println("可以组成的三位数有:" + i + j + k); }

            }

        }


    }
}

}

                                  20211213

//给定任意一个字符串“today is a special day”，长度为任意，要求找出其出现次数最多的字符及计算次数。（可以用HashMap，HashSet，Collections实现）

package haspmap;

import java.awt.print.PrinterGraphics; import java.util.*;

public class hashmap { public static void main(String[] args) { countChar(); }

public static void countChar() {
    Scanner scan = new Scanner(System.in);
    System.out.println("请输入第一行字符串：");
    String str = scan.nextLine();

    HashMap<Character,Integer>map = new HashMap<Character,Integer>();

    for (int i = 0; i < str.length(); i++) {
        Character ch = new Character(str.charAt(i));
        Integer val = new Integer(1);
        Integer oldVal = map.put(ch,val);
        if(oldVal != null){
            map.put(ch,oldVal + 1);
        }
        
    }
    System.out.println(map);
    int max = 0;
    Character ch1 = null;
    Set<Character> set = new HashSet<Character>();
    set = map.keySet();
    Iterator<Character>it = set.iterator();
    while (it.hasNext()){
        Character ch = (Character) it.next();
        Integer pos = map.get(ch).intValue();

        if(pos > max){
            max = pos;
            ch1 = ch;

        }
    }
    System.out.println("字符串" + str + "中出现最多的字符是：" + ch1 + "\n" + "出现次数为：" + max);
            

}

}

输入字符串逆序输出

import java.util.*;

public class java1223 { public static void main(String[] args) { Scanner sc = new Scanner(System.in); String str = sc.next();

    reverseString1(str);
}

private static String reverseString1(String str) {

    char[] chr = str.toCharArray();

    for (int i = chr.length - 1; i >= 0; i--) {
        System.out.print(chr[i]);
    }
    System.out.println("\t");

    return str;
}

}

冒泡排序 public class bubblesort1219 { public static void main(String[] args) { int [] arr = {6,3,8,2,9,1}; System.out.println("排列前的数组为： "); for(int num:arr){ System.out.print(num + " "); } for (int i = 0; i < arr.length-1; i++) { for (int j = 0; j < arr.length-1-i; j++) { if(arr[j]>arr[j+1]){ int tem = arr[j]; arr[j] = arr[j+1]; arr[j+1] = tem; } } } System.out.println(); System.out.println("排序后的数组为： "); for(int num:arr){ System.out.print(num + " "); } } }

二分法 import java.util.Arrays;

public class java1219 { public static void main(String[] args) { int[] arr = {20,34,13,67,12,41,44,69}; int searchWord = 20; Arrays.sort(arr); System.out.println(Arrays.toString(arr)); System.out.println(searchWord + "元素的索引：" + binarySearch(arr,searchWord));

    System.out.println("==========");
    int errorWord = 66;
    System.out.println(errorWord + "元素的索引：" + binarySearch(arr,errorWord));

}


public static int binarySearch(int[] arr, int value){
    int low = 0;
    int high = arr.length - 1;

    while(low <= high){
        int middle = (low + high)/2;

        if(value == arr[middle]){
            return middle;
        }
        if (value > arr[middle]){
            low = middle + 1;
        }
        if (value < arr[middle]){
            high = middle - 1;
        }
    }
    return -1;

}

}