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笔试面试手写代码常见题目，建议收藏，反复观看练习

1…统计一篇英文文章单词个数。

public class WordCounting {   public static void main(String[] args) {         try(FileReader fr = new FileReader("a.txt")) {               int counter = 0;            boolean state = false;            int currentChar;     while((currentChar= fr.read()) != -1) {          if(currentChar== ' ' || currentChar == '\n'          || currentChar == '\t' || currentChar == '\r') {           state = false;       }      else if(!state) {        state = true;     counter++;             }      }      System.out.println(counter);        }      catch(Exception e) {                  e.printStackTrace();     }  }}

补充：这个程序可能有很多种写法，这里选择的是Dennis M. Ritchie和Brian W. Kernighan老师在他们不朽的著作《The C Programming Language》中给出的代码，向两位老师致敬。下面的代码也是如此。

2.输入年月日，计算该日期是这一年的第几天。

import java.util.Scanner;public class DayCounting {       public static void main(String[] args) {           int[][] data = {                   {   31,28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31},                {   31,29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}        };        Scanner sc = new Scanner(System.in);        System.out.print("请输入年月日(1980 11 28): ");        int year = sc.nextInt();        int month = sc.nextInt();        int date = sc.nextInt();        int[] daysOfMonth = data[(year % 4 == 0 && year % 100 != 0 || year % 400 == 0)?1 : 0];        int sum = 0;        for(int i = 0; i < month -1; i++) {               sum += daysOfMonth[i];        }        sum += date;        System.out.println(sum);        sc.close();    }}

3.回文素数：所谓回文数就是顺着读和倒着读一样的数(例如：11，121，1991…)，

public class PalindromicPrimeNumber {       public static void main(String[] args) {           for(int i = 11; i <= 9999; i++) {               if(isPrime(i) && isPalindromic(i)) {                   System.out.println(i);            }        }    }    public static boolean isPrime(int n) {           for(int i = 2; i <= Math.sqrt(n); i++) {               if(n % i == 0) {                   return false;            }        }        return true;    }    public static boolean isPalindromic(int n) {           int temp = n;        int sum = 0;        while(temp > 0) {               sum= sum * 10 + temp % 10;            temp/= 10;        }        return sum == n;    }}

4.全排列：给出五个数字12345的所有排列。

public class FullPermutation {       public static void perm(int[] list) {           perm(list,0);    }    private static void perm(int[] list, int k) {           if (k == list.length) {               for (int i = 0; i < list.length; i++) {                   System.out.print(list[i]);            }            System.out.println();        }else{               for (int i = k; i < list.length; i++) {                   swap(list, k, i);                perm(list, k + 1);                swap(list, k, i);            }        }    }    private static void swap(int[] list, int pos1, int pos2) {           int temp = list[pos1];        list[pos1] = list[pos2];        list[pos2] = temp;    }    public static void main(String[] args) {           int[] x = {   1, 2, 3, 4, 5};        perm(x);    }}

5.对于一个有N个整数元素的一维数组，找出它的子数组（数组中下标连续的元素组成的数组）之和的最大值。

下面给出几个例子（最大子数组用粗体表示）：

public class MaxSum {       private static int max(int x, int y) {           return x > y? x: y;    }    public static int maxSum(int[] array) {           int n = array.length;        int[] start = new int[n];        int[] all = new int[n];        all[n - 1] = start[n - 1] = array[n - 1];        for(int i = n - 2; i >= 0;i--) {               start[i] = max(array[i], array[i] + start[i + 1]);            all[i] = max(start[i], all[i + 1]);        }        return all[0];    }    public static void main(String[] args) {           int[] x1 = {    1, -2, 3, 5,-3, 2 };        int[] x2 = {    0, -2, 3, 5,-1, 2 };        int[] x3 = {    -9, -2, -3,-5, -3 };        System.out.println(maxSum(x1)); // 8        System.out.println(maxSum(x2)); // 9        System.out.println(maxSum(x3)); //-2    }}

6.用递归实现字符串倒转

public class StringReverse {       public static String reverse(String originStr) {           if(originStr == null || originStr.length()== 1) {               return originStr;        }        return reverse(originStr.substring(1))+ originStr.charAt(0);    }    public static void main(String[] args) {           System.out.println(reverse("hello"));    }}

7.输入一个正整数，将其分解为素数的乘积。

import java.util.ArrayList;import java.util.List;import java.util.Scanner;public class DecomposeInteger {       private static List
   
     list = new ArrayList
    
     ();    public static void main(String[] args) {           System.out.print("请输入一个数: ");        Scanner sc = new Scanner(System.in);        int n = sc.nextInt();        decomposeNumber(n);        System.out.print(n + " = ");        for(int i = 0; i < list.size() - 1; i++) {               System.out.print(list.get(i) + " * ");        }        System.out.println(list.get(list.size() - 1));    }    public static void decomposeNumber(int n) {           if(isPrime(n)) {               list.add(n);            list.add(1);        }        else {               doIt(n, (int)Math.sqrt(n));        }    }    public static void doIt(int n, int div) {           if(isPrime(div) && n % div == 0) {               list.add(div);            decomposeNumber(n / div);        }        else {               doIt(n, div - 1);        }    }    public static boolean isPrime(int n) {           for(int i = 2; i <= Math.sqrt(n);i++) {               if(n % i == 0) {                   return false;            }        }        return true;    }}
    
   

8.一个有n级的台阶，一次可以走1级、2级或3级，问走完n级台阶有多少种走法。

public class GoSteps {       public static int countWays(int n) {           if(n < 0) {               return 0;        }        else if(n == 0) {               return 1;        }        else {               return countWays(n - 1) + countWays(n - 2) + countWays(n -3);        }    }    public static void main(String[] args) {           System.out.println(countWays(5)); // 13    }}

9.写一个算法判断一个英文单词的所有字母是否全都不同（不区分大小写）

public class AllNotTheSame {   public static boolean judge(String str) {   String temp = str.toLowerCase();int[] letterCounter = new int[26];for(int i = 0; i 
   
     1) {   return false;}}return true;}public static void main(String[] args) {   System.out.println(judge("hello"));System.out.print(judge("smile"));}}
   

10.有一个已经排好序的整数数组，其中存在重复元素，请将重复元素删除掉，例如，A= [1, 1, 2, 2, 3]，处理之后的数组应当为A= [1, 2, 3]。

public class RemoveDuplication {   public static int[] removeDuplicates(int a[]) {   if(a.length <= 1) {   return a;}int index = 0;for(int i = 1; i < a.length; i++) {   if(a[index] != a[i]) {   a[++index] = a[i];}}int[] b = new int[index + 1];System.arraycopy(a, 0, b, 0, b.length);return b;}public static void main(String[] args) {   int[] a = {   1, 1, 2, 2, 3};a = removeDuplicates(a);System.out.println(Arrays.toString(a));}}

11.给一个数组，其中有一个重复元素占半数以上，找出这个元素。

public class FindMost {   public static 
   
     T find(T[] x){   T temp = null;for(int i = 0, nTimes = 0; i< x.length;i++) {   if(nTimes == 0) {   temp= x[i];nTimes= 1;}else {   if(x[i].equals(temp)) {   nTimes++;}else {   nTimes--;}}}return temp;}public static void main(String[] args) {   String[]strs = {   "hello","kiss","hello","hello","maybe"};System.out.println(find(strs));}}
   

12.编写一个方法求一个字符串的字节长度?

import java.util.Scanner;public class Main {       public static void main(String[] args) {           Scanner sc=  new Scanner(System.in);        while (sc.hasNext()){               int n = sc.nextInt();            double num=1;         while (n!=0){                num = num*n;             n = n-1;         }            System.out.println(Number(num));        }    }    public  static  int Number(double n){           int count=0;        while (n%10==0){               count++;            n = n/10;        }        return count;    }}public int getWordCount(String s){       int length = 0;    for(int i = 0; i < s.length(); i++)    {           int ascii = Character.codePointAt(s, i);        if(ascii >= 0 && ascii <=255)            length++;        else            length += 2;    }    return length;}

总结

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