java基础高级19_String算法题目

/**
 * @author 
 * @description 将一个字符串进行反转。将字符串中指定部分进行反转。
 *              比如“abcdefg”反转为”abfedcg”
 * @create 
 */
public class StringReverse {

    //方式一：转换为char[]
    public String reverse(String str,int startIndex,int endIndex){
        if(str !=null){
            char[] arr = str.toCharArray();
            for (int i = startIndex,j = endIndex; i < j; i++,j--) {
                char temp = arr[i];
                arr[i] = arr[j];
                arr[j] = temp;
            }
            return new String(arr);
        }
        return null;
    }

    //方式二：使用String的拼接
    public String reverse1(String str,int startIndex,int endIndex){
        if(str != null){
            String reverseStr = str.substring(0,startIndex);

            for (int i = endIndex; i >= startIndex; i--) {
                reverseStr += str.charAt(i);
            }
            reverseStr += str.substring(endIndex+1);
            return reverseStr;
        }
        return null;
    }

    //方式三：使用StringBuffer/StringBuilder替换String
    public String reverse2(String str,int startIndex,int endIndex){
        if(str != null){
            StringBuilder builder = new StringBuilder(str.length());
            builder.append(str.substring(0,startIndex));
            for (int i = endIndex; i >= startIndex; i--) {
                builder.append(str.charAt(i));
            }
            builder.append(str.substring(endIndex+1));
            return builder.toString();
        }
        return null;
    }

    @Test
    public void testReverse(){
        String str = "abcdefg";
        String reverse = reverse2(str, 2, 5);
        System.out.println(reverse);
    }
}

/**
 * @author 
 * @description 获取一个字符串在另一个字符串中出现的次数。
 *              比如：获取“ab”在 “abkkcadkabkebfkaabkskab” 中出现的次数
 * 获取subStr在mainStr中出现的次数
 */
public class StringCount {
    public int getCount(String mainStr,String subStr){

        int count = 0;

        int mainLength = mainStr.length();
        int subLength = subStr.length();

        int index = 0;
        if(subLength <= mainLength){

//            while((index = mainStr.indexOf(subStr)) != -1){
//                count++;
//                mainStr = mainStr.substring(index+1);
//            }

            //改进while

            while((index = mainStr.indexOf(subStr,index)) != -1){
                //从index开始找子串
                count++;
//                mainStr = mainStr.substring(index+1);
                index += subLength;
            }

            return count;
        }
        else {return 0;}
    }


    @Test
    public void testGetCount(){
        String mainStr = "abkkcadkabkebfkaabkskab";
        String subStr = "ab";
        int count = getCount(mainStr, subStr);
        System.out.println(count);
    }
}

/**
 * @author 
 * @description 获取两个字符串中最大相同子串
 * str1 = "abcwerthelloyuiodefabcdef";str2 = "cvhellobnm"
 * 提示：将短的那个串进行长度依次递减的子串与较长的串比较。
 */
public class StringMaxS {
    //前提：两个字符串中只有一个最大相同子串
    public String getMaxSameString(String str1,String str2){
        if(str1 != null && str2 != null){
            String maxStr = (str1.length() >= str2.length())? str1 : str2;
            String minStr = (str1.length() < str2.length())? str1 : str2;
            int length = minStr.length();//几大轮

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

                for (int x = 0,y = length - i; y <= length; x++, y++){
                    String subStr = minStr.substring(x, y);
                    if(maxStr.contains(subStr)){
                        return subStr;
                    }
                }

            }

        }
        return null;
    }

    // 如果存在多个长度相同的最大相同子串
    // 此时先返回String[]，后面可以用集合中的ArrayList替换，较方便
    public String[] getMaxSameString1(String str1, String str2) {
        if (str1 != null && str2 != null) {
            StringBuffer sBuffer = new StringBuffer();
            String maxString = (str1.length() > str2.length()) ? str1 : str2;
            String minString = (str1.length() > str2.length()) ? str2 : str1;

            int len = minString.length();
            for (int i = 0; i < len; i++) {
                for (int x = 0, y = len - i; y <= len; x++, y++) {
                    String subString = minString.substring(x, y);
                    if (maxString.contains(subString)) {
                        sBuffer.append(subString + ",");
                    }
                }
//                System.out.println(sBuffer);
                if (sBuffer.length() != 0) {
                    break;
                }
            }
            String[] split = sBuffer.toString().replaceAll(",$", "").split("\,");
            return split;
        }
        return null;
    }


    @Test
    public void testGetMaxSameString(){
        String str1 = "abcwerthello1yuiodefabcdef";
        String str2 = "hellobcdef";
        String maxSameStrings = getMaxSameString(str1, str2);
        System.out.println(maxSameStrings);

    }
}