1. Reverse a string. Reverse the specified part of the string. For example, "abcdefg" is reversed to "abfedcg"
public class String_Change {
//方式一： 转为 char[]
public String reverse1(String str,int startIndex,int endIndex){
if (str!=null){
char[] chars=str.toCharArray();
for(int x = startIndex,y = endIndex;x<y;x++,y--){
char temp = chars[x];
chars[x] = chars[y];
chars[y] = temp;
}
return new String(chars);
}else {
return "";
}
}
// 方式二:截取
public String reverse2(String str ,int startIndex,int endIndex){
if(str !=null){
// 第一部分
String reverseStr = str.substring(0,startIndex);
// 第二部分
for(int i = endIndex;i>=startIndex;i--){
reverseStr +=str.charAt(i);
}
//第三部分
reverseStr +=str.substring(endIndex+1);
return reverseStr;
}else {
return "";
}
}
// 方式三使用 StringBuffer / StringBuilder 替换String
public String reverse3(String str,int startIndex,int endIndex){
if(str !=null){
StringBuilder builder = new StringBuilder(str.length());
// 第一部分
builder.append(str.substring(0,startIndex));
//第二部分
for(int i = endIndex;i>=startIndex;i--){
builder.append(str.charAt(i));
}
//第三部分
builder.append(str.substring(endIndex+1));
return builder.toString();
}
return null;
}
public static void main(String[] args) {
String_Change string_change = new String_Change();
String str="abcdefg";
System.out.println(string_change.reverse1(str, 2, 5));
System.out.println(string_change.reverse2(str, 2, 5));
System.out.println(string_change.reverse3(str, 2, 5));
}
}
2. Get the largest identical substring in two strings. For example:
str1 = "abcwerthelloyuiodefabcdef"; str2 = "cvhellobnm"
Hint: compare the shorter string with the substring of decreasing length and the longer string.
public class String_getMaxSubString {
//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();
for(int i = 0;i < length;i++){
//外层有多少大轮（小字符串的长度）
for(int x = 0,y = length - i;y <= length;x++,y++){
//内层循环相当于一个方框从左移动到右边（动态规划）
String subStr = minStr.substring(x,y);//substring切片要比较的字符串
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) {
//如果找到了该长度的字串，则要中止最外层循环
break;
}
}
String[] split = sBuffer.toString().replaceAll(",$", "").split("\\,");
return split;
}
return null;
}
@Test
public void testGetMaxSameString(){
String str1 = "abcwerthello1yuiodefabcdef";
String str2 = "cvhello1bnmabcdef";
String[] maxSameStrings = getMaxSameString1(str1, str2);
System.out.println(Arrays.toString(maxSameStrings));
}
}
3. Sort the characters in the string in natural order.
Tips:
1) The string becomes an array of characters.
2) Sort the array, select, bubble, Arrays.sort();
3) Turn the sorted array into a string.
public class String_SortString {
public String sortString(String str){
if (str !=null){
char[] chars=str.toCharArray();
//Arrays.sort(chars);//Arrays.sort()
//冒泡排序
for (int i = 0; i < str.length(); i++) {
for (int j = i+1; j < str.length(); j++) {
if (chars[i]>chars[j]){
char temp=chars[i];
chars[i]=chars[j];
chars[j]=temp;
}
}
}
选择排序的优化
// for(int i = 0; i < chars.length - 1; i++) {// 做第i趟排序
// int k = i;
// for(int j = k + 1; j < chars.length; j++){// 选最小的记录
// if(chars[j] < chars[k]){
// k = j; //记下目前找到的最小值所在的位置
// }
// }
// //在内层循环结束，也就是找到本轮循环的最小的数以后，再进行交换
// if(i != k){ //交换a[i]和a[k]
// char temp = chars[i];
// chars[i] = chars[k];
// chars[k] = temp;
// }
// }
return new String(chars);
}
return null;
}
@Test
public void test(){
String_SortString sortString=new String_SortString();
String str="bcgfeda";
String str2=sortString.sortString(str);
System.out.println(str2);
}
}
4. Get the number of times a string occurs in another string. For example: Get the number of occurrences of "ab" in "abkkcadkabkebfkabkskab"
public class String_SubStringTimes {
public int getCount(String mainStr,String subStr){
int mainLength = mainStr.length();
int subLength = subStr.length();
int count = 0;
int index = 0;
if(mainLength >= subLength){
//这两个字符串的地位不一样
//方式一：
// while((index = mainStr.indexOf(subStr)) != -1){
// count++;
// mainStr = mainStr.substring(index + subStr.length());
// }
//方式二：对方式一的改进
while((index = mainStr.indexOf(subStr,index)) != -1){
//index是开始找的位置
count++;
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);
}
}
5. Simulate a trim method to remove spaces at both ends of the string.
public class String_Trim {
public String simulationTrim(String str){
if (str!=null){
int start=0;
int end=str.length()-1;
while (start<end && str.charAt(start)==' '){
start++;
}
while (start<end && str.charAt(end)==' '){
end--;
}
return str.substring(start,end);
}else {
return "";
}
}
public static void main(String[] args) {
String_Trim string_trim=new String_Trim();
String str=" abcdefg ";
System.out.println(string_trim.simulationTrim(str));
}
}