引言：
java中关于String类的常见笔试算法题
- 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”
public class StringDemo {
//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 x = startIndex,y = endIndex;x < y;x++,y--){
char temp = arr[x];
arr[x] = arr[y];
arr[y] = 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){
//第1部分
String reverseStr = str.substring(0,startIndex);
//第2部分
for(int i = endIndex;i >= startIndex;i--){
reverseStr += str.charAt(i);
}
//第3部分
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());
//第1部分
builder.append(str.substring(0,startIndex));
//第2部分
for(int i = endIndex;i >= startIndex;i--){
builder.append(str.charAt(i));
}
//第3部分
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);
}
}- 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
public class StringDemo1 {
public int getCount(String mainStr, String subStr){
int mainLength = mainStr.length();
int subLength = subStr.length();
int count = 0;
int index = 0;
if(mainLength >= subLength){
//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){
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);
}
}- 获取两个字符串中最大相同子串。比如：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 StringDemo2 {
//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++){
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 = "cvhello1bnmabcdef";
String[] maxSameStrings = getMaxSameString1(str1, str2);
System.out.println(Arrays.toString(maxSameStrings));
}
}- 注意以下情况
public class IDEADebug {
@Test
public void testStringBuffer(){
String str = null;
StringBuffer sb = new StringBuffer();//StringBuilder与之类似
sb.append(str);//
System.out.println(sb.length());//4
System.out.println(sb);//"null"
StringBuffer sb1 = new StringBuffer(str);//抛异常NullPointerException
System.out.println(sb1);//
}
}
