算法-kmp

2022-3-16 diaba 算法

package com.jiucaiyuan.net.algrithm.sum;

/**
 * Knuth-Morris-Pratt
 *
 * 
 */
class Kmp {

    /**
     * kmp算法
     * @param haystack
     * @param needle
     * @return
     */
    public static int strStr(String haystack, String needle) {

        int n = haystack.length(), m = needle.length();
        if (m == 0) {
            return 0;
        }

        int[] pi = new int[m];
        for (int i = 1, j = 0; i < m; i++) {
            while (j > 0 && needle.charAt(i) != needle.charAt(j)) {
                j = pi[j - 1];
            }
            if (needle.charAt(i) == needle.charAt(j)) {
                j++;
            }
            pi[i] = j;
        }

        for (int i = 0, j = 0; i < n; i++) {
            while (j > 0 && haystack.charAt(i) != needle.charAt(j)) {
                j = pi[j - 1];
            }
            if (haystack.charAt(i) == needle.charAt(j)) {
                j++;
            }
            if (j == m) {
                return i - m + 1;
            }
        }

        return -1;

    }

    /**
     * 暴力破解
     * @param haystack
     * @param needle
     * @return
     */
    public static int strStr2(String haystack, String needle) {
        int n = haystack.length(), m = needle.length();
        for (int i = 0; i + m <= n; i++) {
            boolean flag = true;
            for (int j = 0; j < m; j++) {
                if (haystack.charAt(i + j) != needle.charAt(j)) {
                    flag = false;
                    break;
                }
            }
            if (flag) {
                return i;
            }
        }
        return -1;
    }


    public static void main(String[] args) {
        String haystack = "abcdefg";
        String needle = "ef";
        System.out.println("1 "+haystack + "-" + needle + "-" + strStr(haystack, needle));
        System.out.println("2 "+haystack + "-" + needle + "-" + strStr2(haystack, needle));
        haystack = "abcdefghijklmnabcdefghijklmnopqrst";
        needle = "abcdefghijklmnop";
        System.out.println("1 "+haystack + "-" + needle + "-" + strStr(haystack, needle));
        System.out.println("2 "+haystack + "-" + needle + "-" + strStr2(haystack, needle));
    }

}

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