kmp algorithm java code example

Example 1: kmp java

static int KMPSearch(String pat, String txt) {
        ArrayList<Integer> matchAtIndexs = new ArrayList<>();
        int M = pat.length();
        int N = txt.length();
        int lps[] = new int[M];
        int j = 0;
        computeLPSArray(pat, M, lps);
        int i = 0;
        while (i < N) {
            if (pat.charAt(j) == txt.charAt(i)) {
                j++;
                i++;
            }
            if (j == M) {
                matchAtIndexs.add(i - j);
                j = lps[j - 1];
            } else if (i < N && pat.charAt(j) != txt.charAt(i)) {
                if (j != 0)
                    j = lps[j - 1];
                else
                    i = i + 1;
            }
        }
        return matchAtIndexs.size();
    }

    static void computeLPSArray(String pat, int M, int lps[]) {
        int len = 0;
        int i = 1;
        lps[0] = 0;
        while (i < M) {
            if (pat.charAt(i) == pat.charAt(len)) {
                len++;
                lps[i] = len;
                i++;
            } else {
                if (len != 0) {
                    len = lps[len - 1];

                } else {
                    lps[i] = len;
                    i++;
                }
            }
        }
    }

Example 2: kmp java

ArrayList<Integer> matachedAtindex;

    void KMPSearch(char[] pat, char[] txt) {
        int m = pat.length;
        int n = txt.length;
        int j = 0;
        int i = 0;
        matachedAtindex = new ArrayList<>();
        lpsCompute(pat);
        while (i < n) {
            if (pat[j] == txt[i]) {
                j++;
                i++;
            }
            if (j == m) {
                matachedAtindex.add((i - j));
                j = lps[j - 1];
            } else if (i < n && pat[j] != txt[i]) {
                if (j != 0)
                    j = lps[j - 1];
                else
                    i = i + 1;
            }
        }
    }
    
    static int lps[];

    static void lpsCompute(char[] pat) {
        int n = pat.length;
        lps = new int[n];
        int j = 0;
        int i = 1;
        while (i < n) {
            if (pat[i] == pat[j]) {
                j++;
                lps[i] = j;
                i++;
            } else {
                if (j != 0) {
                    j = lps[j - 1];
                } else {
                    lps[i] = j;
                    i++;
                }
            }
        }
    }