原文地址: https://caoxudong818.iteye.com/blog/1218109
用Java实现KMP算法，并与String.indexOf方法做比较。
public static int kmp1(String pattern, String src){
int srcLength = src.length() ;
int patternLength = pattern.length() ;
int index = -1 ;
int count = 0 ;
for(int i = 0 ; i < srcLength - 1 ; i++){
count = 0 ;
for(int j = 0 ; (j < patternLength ) && (i < srcLength) ; i++){
if(src.charAt(i) == pattern.charAt(j)){
count++ ;
i++ ;
}else{
break ;
}
}
if(count == patternLength){
return i - count ;
}
if(count == 0){
i++ ;
}
}
return index ;
}
测试程序如下：
public static void main(String[] args){
String src = "1234567890abcdefghijk" ;
String pattern = "abcd" ;
long beginTime = System.nanoTime() ;
int index = kmp1(pattern, src) ;
System.out.println("My KMP : index = " + index + " and time = " + (System.nanoTime() - beginTime) + "ns");
beginTime = System.nanoTime() ;
index = src.indexOf(pattern) ;
System.out.println("String.indexOf : index = " + index + " and time = " + (System.nanoTime() - beginTime) + "ns");
}
结果很悲剧，输出如下
My KMP : index = 10 and time = 58012 ns
String.indexOf : index = 10 and time = 9895 ns
对代码进行修改，发现方法kmp1在内层循环中对条件判断的使用有些费操作，遂修改如下：
public static int kmp2(String pattern, String src){
int srcLength = src.length() ;
int patternLength = pattern.length() ;
int index = -1 ;
int count = 0 ;
for(int i = 0 ; i < srcLength - 1 ; i++){
count = 0 ;
for(int j = 0 ; (j < patternLength ) && (i < srcLength) && (src.charAt(i) == pattern.charAt(j)); j++,i++,count++) ;
if(count == patternLength){
return i - count ;
}
if(count == 0){
i++ ;
}
}
return index ;
}
执行后发现没什么效果，输出如下：
My KMP : index = 10 and time = 57070 ns
String.indexOf : index = 10 and time = 9961 ns
再将字符串改为对字符数组的操作
public static int kmp3(char[] patternArray , char[] srcArray){
int index = -1 ;
int count = 0 ;
for(int i = 0 ; i < srcArray.length - 1 ; i++){
count = 0 ;
for(int j = 0 ; (j < patternArray.length ) && (i < srcArray.length) && (srcArray[i] == patternArray[j]); j++,i++,count++) ;
if(count == patternArray.length){
return i - count ;
}
if(count == 0){
i++ ;
}
}
return index ;
}
效果依然不好：
My KMP : index = 10 and time = 55805 ns
String.indexOf : index = 10 and time = 10277 ns
各位看官，给点提醒吧，还可以从哪里优化吗？还是说，我写的KMP本身就有问题？