java笔试手写算法面试题大全含答案

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

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;

}