java笔试手写算法面试题大全含答案

Dear 丶 2023-10-05 19:53 40阅读 0赞

**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。**  
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…)，回文素数就是既是回文数又是素数(只能被1和自身整除的数)的数。编程找出11～9999之间的回文素数。**  
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）：  
数组：\{ 1, -2, 3,5, -3, 2 \}，结果是：8  
2) 数组：\{ 0, -2, 3, 5, -1, 2 \}，结果是：9  
3) 数组：\{ -9, -2,-3, -5, -3 \}，结果是：-2  
可以使用动态规划的思想求解：  
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。**  
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 <temp.length(); i++) \{  
int index = temp.charAt(i)- ‘a’;  
letterCounter\[index\]++;  
if(letterCounter\[index\] > 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?**  
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;  
\}