关于KMP算法
Posted jieyuefeng
tags:
篇首语:本文由小常识网(cha138.com)小编为大家整理,主要介绍了关于KMP算法相关的知识,希望对你有一定的参考价值。
KMP算法的重点在于:当某一个字符与主串不匹配时,我们应该知道把指针j移动到哪里
// next[j] = k,表示当T[i] != P[j]时,j指针的下一个位置
public static int[] getNext(String ps) {
char[] p = ps.toCharArray();
int[] next = new int[p.length];
next[0] = -1;
int j = 0;
int k = -1;
while (j < p.length - 1) {
if (k == -1 || p[j] == p[k]) {
next[++j] = ++k;
} else {
k = next[k];
}
}
return next;
}
鉴于KMP算法略晦涩,所以去看了Java中String的indexOf方法,意外发现居然是暴力破解法:
/**
* Code shared by String and StringBuffer to do searches. The
* source is the character array being searched, and the target
* is the string being searched for.
*
* @param source the characters being searched.
* @param sourceOffset offset of the source string.
* @param sourceCount count of the source string.
* @param target the characters being searched for.
* @param targetOffset offset of the target string.
* @param targetCount count of the target string.
* @param fromIndex the index to begin searching from.
*/
static int indexOf(char[] source, int sourceOffset, int sourceCount,
char[] target, int targetOffset, int targetCount,
int fromIndex) {
if (fromIndex >= sourceCount) {
return (targetCount == 0 ? sourceCount : -1);
}
if (fromIndex < 0) {
fromIndex = 0;
}
if (targetCount == 0) {
return fromIndex;
}
char first = target[targetOffset];
int max = sourceOffset + (sourceCount - targetCount);
for (int i = sourceOffset + fromIndex; i <= max; i++) {
/* Look for first character. */
if (source[i] != first) {
while (++i <= max && source[i] != first);
}
/* Found first character, now look at the rest of v2 */
if (i <= max) {
int j = i + 1;
int end = j + targetCount - 1;
for (int k = targetOffset + 1; j < end && source[j]
== target[k]; j++, k++);
if (j == end) {
/* Found whole string. */
return i - sourceOffset;
}
}
}
return -1;
}
于是又百度了一下,这样做可能的原因如下:
原来JDK的编写者们认为大多数情况下,字符串都不长,使用原始实现可能代价更低。因为KMP和Boyer-Moore算法都需要预先计算处理来获得辅助数组,需要一定的时间和空间,这可能在短字符串查找中相比较原始实现耗费更大的代价。而且一般大字符串查找时,程序员们也会使用其它特定的数据结构,查找起来更简单。这有点类似于排除特定情况下的快速排序了。不同环境选择不同算法。
以上是关于关于KMP算法的主要内容,如果未能解决你的问题,请参考以下文章