[java实现]常见算法之字符串操作

一、字符串反转

把一个句子中的打次进行反转，比如“how are you” ,变为 “you are how”

// 字符串反转
public class StringTest {

    // 字符反转的方法
    private void swap(char[] c, int front, int end) {

        if (front > end || end >= c.length) {
            return;
        }

        while (front < end) {

            char tmp = c[front];
            c[front] = c[end];
            c[end] = tmp;

            front++;
            end--;
        }
    }

    // O(n)
    public String swapStr(String str) {

        char[] cArr = str.toCharArray();

        // 整个字符串的字符反转
        swap(cArr, 0, cArr.length - 1); // 反转整个字符串中的所有字母，how are you -> uoy era woh

        int begin = 0;

        // 对字符串中的每个单词反转，除了最后一单词
        for (int i = 0; i < cArr.length; i++) {

            if (cArr[i] == ' ') {
                swap(cArr, begin, i - 1);
                begin = i + 1;
            }
        }

        // 最后一个单词的反转
        swap(cArr, begin, cArr.length - 1);

        return new String(cArr);
    }

    public static void main(String[] args) {

        String str = "how are you";
        System.out.println(new StringTest().swapStr(str));
    }

}

二、判断字符串是否由相同的字符组成

判断连个字符串的字母个字母的个数是否一样，顺序可以不同， 如“aaaabc” 和“cbaaaa”是相同的

// 判断字符串是否由相同的字符组成
public class StringTest {

    // 方法一 可以死任意字符      O(nlogn)
    public boolean compareStr(String str1, String str2) {

        byte[] bs1 = str1.getBytes();
        byte[] bs2 = str2.getBytes();

        Arrays.sort(bs1);
        Arrays.sort(bs2);

        str1 = new String(bs1);
        str2 = new String(bs2);

        if (str1.equals(str2)) {
            return true;
        } else {
            return false;
        }
    }

    // 只能是ASCII码  方法二  O(n)
    public boolean compareStr2(String str1, String str2) {

        byte[] bs1 = str1.getBytes();
        byte[] bs2 = str2.getBytes();

        int bCount[] = new int[256];

        for (int i = 0; i < bs1.length; i++)
            bCount[bs1[i] ]++;
        for (int i = 0; i < bs2.length; i++)
            bCount[bs2[i] ]--;
        for (int i = 0; i < 256; i++) {

            if (bCount[i] != 0) {
                return false;
            }

        }
        return true;

    }

    public static void main(String[] args) {

        String str1 = "aaaabbc";
        String str2 = "cbaaaab";

        System.out.println(new StringTest().compareStr2(str1, str2));

    }

}

三、字符串中单词的统计

给定一段空格分开的字符串，判断单词的个数

// 字符串中单词的统计
public class StringTest {

    // O(n)
    public int wordCount(String str) {

        int word = 0;
        int count = 0;
        for (int i = 0; i < str.length(); i++) {

            if (str.charAt(i) == ' ')
                word = 0;
            else if (word == 0 ) {
                word = 1;
                count++;
            }
        }

        return count;
    }

    public static void main(String[] args) {

        String str = "i am a good boy";

        System.out.println(new StringTest().wordCount(str));

    }

}

四、删除字符串中重复的字符

删除字符串中国重复的字符。如good -> god

// 删除字符串中重复的字符
public class StringTest {

    // O(n^2)
    public String removeDuplicate(String str) {

        char[] cs = str.toCharArray();
        int n = cs.length;

        for (int i = 0; i < n; i++) {

            if (cs[i] == '\0')
                continue;

            for (int j = i + 1; j < n; j++) {

                if (cs[j] == '\0')
                    continue;
                if (cs[i] == cs[j])
                    cs[j] = '\0';
            }
        }

        int be = 0;
        for (int i = 0; i < n; i++) {
            if (cs[i] != '\0')
                cs[be++] =cs[i];
        }

        return new String(cs, 0, be);
    }

    // 方法二： O(n)
    public String removeDuplicate2(String str) {

        char[] cs = str.toCharArray();
        int n = cs.length;

        int count[] = new int[256];
        for (int i = 0; i < cs.length; i++) {
            if (count[cs[i]] != 0)
                cs[i] = '\0';
            count[cs[i]]++;
        }

        int be = 0;
        for (int i = 0; i < n; i++) {
            if (cs[i] != '\0')
                cs[be++] = cs[i];
        }

        return new String(cs, 0, be);
    }

    public static void main(String[] args) {

        String str = "aaaabbc";

        System.out.println(new StringTest().removeDuplicate(str));

    }

}

五、按要求打印给定数组的排列情况

如1，2,2,3,4,5，要求第四位不为4,3和5不能相连

// 按要求打印数组的排列情况
import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

public class StringTest {

    boolean visited[];
    String combination = "";
    int graph[][] = null;

    public void getAllCombination(int arr[]) {

        int n = arr.length;
        graph = new int[n][n];
        visited = new boolean[n];

        buildGraph(arr, graph);

        Set<String> set = new HashSet<>();

        for (int i = 0; i < n; i++) {
            depthFirstSearch(i, set, arr);
        }

        Iterator<String> iterator = set.iterator();

        while (iterator.hasNext()) {
            String string = (String) iterator.next();
            System.out.println(string);
        }
    }

    /**
     * 按照深度优先遍历 图，将符合要求的组合加入到set中，自动去重
     *
     * @param start
     * @param set
     * @param arr
     */
    private void depthFirstSearch(int start, Set<String> set, int arr[]) {
        visited[start] = true;
        combination += arr[start];

        if (combination.length() == arr.length) {
            if (combination.indexOf("4") != 2) {
                set.add(combination);
            }
        }

        for (int j = 0; j < arr.length; j++) {
            if (graph[start][j] == 1 && visited[j] == false)
                depthFirstSearch(j, set, arr);
        }

        // 什么意思?
        combination = combination.substring(0, combination.length() - 1);
        visited[start] = false;
    }

    /**
     * 根据传入的数构建一个图，图中的 3,5 不能相连
     *
     * @param arr
     * @param graph
     * @return
     */
    private int[][] buildGraph(int arr[], int[][] graph) {

        for (int i = 0; i < arr.length; i++) {
            for (int j = 0; j < arr.length; j++) {
                if (arr[i] == arr[j])
                    graph[i][j] = 0;
                else
                    graph[i][j] = 1;
            }
        }

        graph[3][5] = 0;
        graph[5][3] = 0;

        return graph;
    }

    public static void main(String[] args) {

        int arr[] = { 1, 2, 2, 3, 4, 5 };
        new StringTest().getAllCombination(arr);

    }

}

六、输出字符串的所有组合

给定一个字符串，输出该字符串中字符的所有组合

// 输出字符串的所有组合

public class StringTest {

    public void combineRecursive(char[] c, int begin, int len, StringBuffer sb) {

        if (len == 0) {
            System.out.print(sb + " ");
            return;
        }

        if (begin == c.length)
            return;
        sb.append(c[begin]);
        combineRecursive(c, begin + 1, len - 1, sb);
        sb.deleteCharAt(sb.length() - 1);
        combineRecursive(c, begin + 1, len, sb);

    }

    public static void main(String[] args) {

        String s = "abc";
        char[] cs = s.toCharArray();
        StringBuffer sb = new StringBuffer();

        for (int j = 1; j < cs.length; j++) {
            new StringTest().combineRecursive(cs, 0, j, sb);
        }
    }

}