我对此进行了大量搜索，并且大多数帖子都在谈论在两个arraylists之间寻找可以用Collections.retainAll完成的公共字符串或者包含与文本进行比较的单个单词的ArrayList。
我的文字可能在Java中看起来像这样。
String text = "Get a placement right today by applying to our interviews and don't forget to email us your resume. This is a top job opportunity to get yourself acquainted with real world programming and skill building. Hurry! apply for placement now here";
我有一个ArrayList，可以说2个字符串，“placement”和“job opportunity”
我希望结果为展示位置（2）和工作机会（1） 我目前有几种方法，但我想知道实现这一目标的最佳方法。
方法1 为ArrayList中的每个单词维护一个计数器。对于ArrayList中的每个单词，执行text.contains（word），如果为true，则递增相应的计数器，如果文本中的单词多于ArrayList，或者ArrayList中的单词多于此处的文本，会发生什么？是否有任何最佳或更短的方法来实现相同的目标？我的ArrayList中可能有单词或短语。提前感谢您的建议。
答案 0 :(得分：3)
一种简单的方法是使用String.indexOf搜索列表中的每个单词：
for (String word : list) {
int prev = -1;
int count = 0;
do {
prev = s.indexOf(word, prev + 1);
if (prev != -1 /* && check for word breaks */) {
count++
}
} while (prev != -1);
System.out.println(word + " " + count);
}
然而，除了简单性之外，这并不是针对任何特定标准而设计的。
请注意，这不会检查分词，因此会在"foo"中找到"xfoox";有可能改变我指示寻找这些条件的条件。
如果您需要处理非常大的单词列表，像Aho-Corasick这样的算法会更有效，因为这样可以避免检查列表中的所有字符串。但是，它需要对单词列表进行一些预处理，尽管这可以合理有效地实现，并且可以在离线时针对给定的单词列表完成。
答案 1 :(得分：2)
如果我理解正确，这个问题就是模式匹配问题的一个例子。 This wikipedia page列出了最佳字符串搜索算法及其平均和最差情况的复杂性。 如果我没记错的话，Alfred V. Aho设计和分析算法，Jerffery Ullman和John E. Hopcroft对模式匹配章节中的Finite-state automaton based search进行了分析。
以下两个似乎效率最高。
我在http://algs4.cs.princeton.edu找到了这两种算法的实现 如果链接断开，我也会在这里复制文件。 实现：
StdOut 只是 System.out
备份KMP：
/******************************************************************************
* Compilation: javac KMP.java
* Execution: java KMP pattern text
* Dependencies: StdOut.java
*
* Reads in two strings, the pattern and the input text, and
* searches for the pattern in the input text using the
* KMP algorithm.
*
* % java KMP abracadabra abacadabrabracabracadabrabrabracad
* text: abacadabrabracabracadabrabrabracad
* pattern: abracadabra
*
* % java KMP rab abacadabrabracabracadabrabrabracad
* text: abacadabrabracabracadabrabrabracad
* pattern: rab
*
* % java KMP bcara abacadabrabracabracadabrabrabracad
* text: abacadabrabracabracadabrabrabracad
* pattern: bcara
*
* % java KMP rabrabracad abacadabrabracabracadabrabrabracad
* text: abacadabrabracabracadabrabrabracad
* pattern: rabrabracad
*
* % java KMP abacad abacadabrabracabracadabrabrabracad
* text: abacadabrabracabracadabrabrabracad
* pattern: abacad
*
******************************************************************************/
/**
* The <tt>KMP</tt> class finds the first occurrence of a pattern string
* in a text string.
* <p>
* This implementation uses a version of the Knuth-Morris-Pratt substring search
* algorithm. The version takes time as space proportional to
* <em>N</em> + <em>M R</em> in the worst case, where <em>N</em> is the length
* of the text string, <em>M</em> is the length of the pattern, and <em>R</em>
* is the alphabet size.
* <p>
* For additional documentation,
* see <a href="http://algs4.cs.princeton.edu/53substring">Section 5.3</a> of
* <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne.
*/
public class KMP {
private final int R; // the radix
private int[][] dfa; // the KMP automoton
private char[] pattern; // either the character array for the pattern
private String pat; // or the pattern string
/**
* Preprocesses the pattern string.
*
* @param pat the pattern string
*/
public KMP(String pat) {
this.R = 256;
this.pat = pat;
// build DFA from pattern
int M = pat.length();
dfa = new int[R][M];
dfa[pat.charAt(0)][0] = 1;
for (int X = 0, j = 1; j < M; j++) {
for (int c = 0; c < R; c++)
dfa[c][j] = dfa[c][X]; // Copy mismatch cases.
dfa[pat.charAt(j)][j] = j+1; // Set match case.
X = dfa[pat.charAt(j)][X]; // Update restart state.
}
}
/**
* Preprocesses the pattern string.
*
* @param pattern the pattern string
* @param R the alphabet size
*/
public KMP(char[] pattern, int R) {
this.R = R;
this.pattern = new char[pattern.length];
for (int j = 0; j < pattern.length; j++)
this.pattern[j] = pattern[j];
// build DFA from pattern
int M = pattern.length;
dfa = new int[R][M];
dfa[pattern[0]][0] = 1;
for (int X = 0, j = 1; j < M; j++) {
for (int c = 0; c < R; c++)
dfa[c][j] = dfa[c][X]; // Copy mismatch cases.
dfa[pattern[j]][j] = j+1; // Set match case.
X = dfa[pattern[j]][X]; // Update restart state.
}
}
/**
* Returns the index of the first occurrrence of the pattern string
* in the text string.
*
* @param txt the text string
* @return the index of the first occurrence of the pattern string
* in the text string; N if no such match
*/
public int search(String txt) {
// simulate operation of DFA on text
int M = pat.length();
int N = txt.length();
int i, j;
for (i = 0, j = 0; i < N && j < M; i++) {
j = dfa[txt.charAt(i)][j];
}
if (j == M) return i - M; // found
return N; // not found
}
/**
* Returns the index of the first occurrrence of the pattern string
* in the text string.
*
* @param text the text string
* @return the index of the first occurrence of the pattern string
* in the text string; N if no such match
*/
public int search(char[] text) {
// simulate operation of DFA on text
int M = pattern.length;
int N = text.length;
int i, j;
for (i = 0, j = 0; i < N && j < M; i++) {
j = dfa[text[i]][j];
}
if (j == M) return i - M; // found
return N; // not found
}
/**
* Takes a pattern string and an input string as command-line arguments;
* searches for the pattern string in the text string; and prints
* the first occurrence of the pattern string in the text string.
*/
public static void main(String[] args) {
String pat = args[0];
String txt = args[1];
char[] pattern = pat.toCharArray();
char[] text = txt.toCharArray();
KMP kmp1 = new KMP(pat);
int offset1 = kmp1.search(txt);
KMP kmp2 = new KMP(pattern, 256);
int offset2 = kmp2.search(text);
// print results
StdOut.println("text: " + txt);
StdOut.print("pattern: ");
for (int i = 0; i < offset1; i++)
StdOut.print(" ");
StdOut.println(pat);
StdOut.print("pattern: ");
for (int i = 0; i < offset2; i++)
StdOut.print(" ");
StdOut.println(pat);
}
}
备份Boyer Moore：
BoyerMoore.java
Below is the syntax highlighted version of BoyerMoore.java from §5.3 Substring Search.
/******************************************************************************
* Compilation: javac BoyerMoore.java
* Execution: java BoyerMoore pattern text
* Dependencies: StdOut.java
*
* Reads in two strings, the pattern and the input text, and
* searches for the pattern in the input text using the
* bad-character rule part of the Boyer-Moore algorithm.
* (does not implement the strong good suffix rule)
*
* % java BoyerMoore abracadabra abacadabrabracabracadabrabrabracad
* text: abacadabrabracabracadabrabrabracad
* pattern: abracadabra
*
* % java BoyerMoore rab abacadabrabracabracadabrabrabracad
* text: abacadabrabracabracadabrabrabracad
* pattern: rab
*
* % java BoyerMoore bcara abacadabrabracabracadabrabrabracad
* text: abacadabrabracabracadabrabrabracad
* pattern: bcara
*
* % java BoyerMoore rabrabracad abacadabrabracabracadabrabrabracad
* text: abacadabrabracabracadabrabrabracad
* pattern: rabrabracad
*
* % java BoyerMoore abacad abacadabrabracabracadabrabrabracad
* text: abacadabrabracabracadabrabrabracad
* pattern: abacad
*
******************************************************************************/
/**
* The <tt>BoyerMoore</tt> class finds the first occurrence of a pattern string
* in a text string.
* <p>
* This implementation uses the Boyer-Moore algorithm (with the bad-character
* rule, but not the strong good suffix rule).
* <p>
* For additional documentation,
* see <a href="http://algs4.cs.princeton.edu/53substring">Section 5.3</a> of
* <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne.
*/
public class BoyerMoore {
private final int R; // the radix
private int[] right; // the bad-character skip array
private char[] pattern; // store the pattern as a character array
private String pat; // or as a string
/**
* Preprocesses the pattern string.
*
* @param pat the pattern string
*/
public BoyerMoore(String pat) {
this.R = 256;
this.pat = pat;
// position of rightmost occurrence of c in the pattern
right = new int[R];
for (int c = 0; c < R; c++)
right[c] = -1;
for (int j = 0; j < pat.length(); j++)
right[pat.charAt(j)] = j;
}
/**
* Preprocesses the pattern string.
*
* @param pattern the pattern string
* @param R the alphabet size
*/
public BoyerMoore(char[] pattern, int R) {
this.R = R;
this.pattern = new char[pattern.length];
for (int j = 0; j < pattern.length; j++)
this.pattern[j] = pattern[j];
// position of rightmost occurrence of c in the pattern
right = new int[R];
for (int c = 0; c < R; c++)
right[c] = -1;
for (int j = 0; j < pattern.length; j++)
right[pattern[j]] = j;
}
/**
* Returns the index of the first occurrrence of the pattern string
* in the text string.
*
* @param txt the text string
* @return the index of the first occurrence of the pattern string
* in the text string; N if no such match
*/
public int search(String txt) {
int M = pat.length();
int N = txt.length();
int skip;
for (int i = 0; i <= N - M; i += skip) {
skip = 0;
for (int j = M-1; j >= 0; j--) {
if (pat.charAt(j) != txt.charAt(i+j)) {
skip = Math.max(1, j - right[txt.charAt(i+j)]);
break;
}
}
if (skip == 0) return i; // found
}
return N; // not found
}
/**
* Returns the index of the first occurrrence of the pattern string
* in the text string.
*
* @param text the text string
* @return the index of the first occurrence of the pattern string
* in the text string; N if no such match
*/
public int search(char[] text) {
int M = pattern.length;
int N = text.length;
int skip;
for (int i = 0; i <= N - M; i += skip) {
skip = 0;
for (int j = M-1; j >= 0; j--) {
if (pattern[j] != text[i+j]) {
skip = Math.max(1, j - right[text[i+j]]);
break;
}
}
if (skip == 0) return i; // found
}
return N; // not found
}
/**
* Takes a pattern string and an input string as command-line arguments;
* searches for the pattern string in the text string; and prints
* the first occurrence of the pattern string in the text string.
*/
public static void main(String[] args) {
String pat = args[0];
String txt = args[1];
char[] pattern = pat.toCharArray();
char[] text = txt.toCharArray();
BoyerMoore boyermoore1 = new BoyerMoore(pat);
BoyerMoore boyermoore2 = new BoyerMoore(pattern, 256);
int offset1 = boyermoore1.search(txt);
int offset2 = boyermoore2.search(text);
// print results
StdOut.println("text: " + txt);
StdOut.print("pattern: ");
for (int i = 0; i < offset1; i++)
StdOut.print(" ");
StdOut.println(pat);
StdOut.print("pattern: ");
for (int i = 0; i < offset2; i++)
StdOut.print(" ");
StdOut.println(pat);
}
}
Copyright © 2002–2015, Robert Sedgewick and Kevin Wayne.
Last updated: Sat Aug 29 11:16:30 EDT 2015.