1.統(tǒng)計(jì)一篇英文文章單字?jǐn)?shù)。

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();
        }
    }
}

補(bǔ)充：這個(gè)程式可能有很多種寫(xiě)法，這裡選擇的是Dennis M. Ritchie和Brian W. Kernighan老師在他們不朽的著作《The C Programming Language》中給出的代碼，向兩位老師致敬。下面的程式碼也是如此。

2.輸入年月日，計(jì)算該日期是這一年的第幾天。

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("請(qǐng)輸入年月日(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.回文質(zhì)數(shù)：所謂回文數(shù)就是順著讀和倒著讀一樣的數(shù)(例如：11，121，1991…)，回文素?cái)?shù)就是既是回文數(shù)又是質(zhì)數(shù)(只能被1和自身整除的數(shù))的數(shù)。程式找出11～9999之間的回文素?cái)?shù)。

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：給出五個(gè)數(shù)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.對(duì)於一個(gè)有N個(gè)整數(shù)元素的一維數(shù)組，找出它的子數(shù)組（數(shù)組中下標(biāo)連續(xù)的元素所組成的數(shù)組）之和的最大值。

下面給幾個(gè)例子（最大子陣列以粗體表示）：

陣列：{ 1, -2, 3, 5, -3, 2 }，結(jié)果是：8

2) 陣列：{ 0, -2, 3, 5, -1, 2 }，結(jié)果是：9

3)陣列：{ -9, -2,-3, -5, -3 }，結(jié)果是：-2

可以使用動(dòng)態(tài)規(guī)劃的想法來(lái)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.用遞迴實(shí)作字串倒轉(zhuǎn)

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.輸入正整數(shù)，將其分解為質(zhì)數(shù)的乘積。

public class DecomposeInteger {
    private static List<Integer> list = new ArrayList<Integer>();
    public static void main(String[] args) {
        System.out.print("請(qǐng)輸入一個(gè)數(shù): ");
        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、一個(gè)有n級(jí)的臺(tái)階，一次可以走1級(jí)、2級(jí)或3級(jí)，問(wèn)走完n級(jí)臺(tái)階有多少種走法。

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.寫(xiě)一個(gè)演算法判斷一個(gè)英文單字的所有字母是否全都不同（不區(qū)分大小寫(xiě)）

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)- &#39;a&#39;;
            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.有一個(gè)已經(jīng)排好序的整數(shù)數(shù)組，其中存在重複元素，請(qǐng)將重複元素刪除掉，例如，A= [1, 1, 2, 2, 3]，處理之後的數(shù)組應(yīng)當(dāng)為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.給一個(gè)數(shù)組，其中有一個(gè)重複元素佔(zhàn)半數(shù)以上，找出這個(gè)元素。

public class FindMost {
    public static <T> 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.寫(xiě)一個(gè)方法求一個(gè)字串的位元組長(zhǎng)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;
}

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