用于模式搜索的 KMP 算法的Java程序

给定一个文本txt[0..n-1]和一个模式pat[0..m-1] ，编写一个函数search(char pat[], char txt[])打印txt中所有出现的pat[] [] 。您可以假设n > m 。

例子：

输入：txt[] = "THIS IS A TEST TEXT" pat[] = "TEST" 输出：在索引 10 找到的模式 输入：txt[] = "AABAACAADAABAABA" pat[] = "AABA" 输出：在索引 0 找到的模式在索引 9 找到的模式 在索引 12 找到的模式

模式搜索是计算机科学中的一个重要问题。当我们在记事本/word 文件或浏览器或数据库中搜索字符串时，会使用模式搜索算法来显示搜索结果。

Java

// JAVA program for implementation of KMP pattern
// searching algorithm
  
class KMP_String_Matching {
    void KMPSearch(String pat, String txt)
    {
        int M = pat.length();
        int N = txt.length();
  
        // create lps[] that will hold the longest
        // prefix suffix values for pattern
        int lps[] = new int[M];
        int j = 0; // index for pat[]
  
        // Preprocess the pattern (calculate lps[]
        // array)
        computeLPSArray(pat, M, lps);
  
        int i = 0; // index for txt[]
        while (i < N) {
            if (pat.charAt(j) == txt.charAt(i)) {
                j++;
                i++;
            }
            if (j == M) {
                System.out.println("Found pattern "
                                   + "at index " + (i - j));
                j = lps[j - 1];
            }
  
            // mismatch after j matches
            else if (i < N && pat.charAt(j) != txt.charAt(i)) {
                // Do not match lps[0..lps[j-1]] characters,
                // they will match anyway
                if (j != 0)
                    j = lps[j - 1];
                else
                    i = i + 1;
            }
        }
    }
  
    void computeLPSArray(String pat, int M, int lps[])
    {
        // length of the previous longest prefix suffix
        int len = 0;
        int i = 1;
        lps[0] = 0; // lps[0] is always 0
  
        // the loop calculates lps[i] for i = 1 to M-1
        while (i < M) {
            if (pat.charAt(i) == pat.charAt(len)) {
                len++;
                lps[i] = len;
                i++;
            }
            else // (pat[i] != pat[len])
            {
                // This is tricky. Consider the example.
                // AAACAAAA and i = 7. The idea is similar
                // to search step.
                if (len != 0) {
                    len = lps[len - 1];
  
                    // Also, note that we do not increment
                    // i here
                }
                else // if (len == 0)
                {
                    lps[i] = len;
                    i++;
                }
            }
        }
    }
  
    // Driver program to test above function
    public static void main(String args[])
    {
        String txt = "ABABDABACDABABCABAB";
        String pat = "ABABCABAB";
        new KMP_String_Matching().KMPSearch(pat, txt);
    }
}
// This code has been contributed by Amit Khandelwal.

输出：

Found pattern at index 10

有关更多详细信息，请参阅关于模式搜索的 KMP 算法的完整文章！