Java实现KMP算法

直接上算法实现及测试代码

public class KMP {

    public static int getIndexOf(String s1, String s2) {
        if (s1 == null || s2 == null || s2.isEmpty() || s1.length() < s2.length()) {
            return -1;
        }
        char[] str1 = s1.toCharArray();
        char[] str2 = s2.toCharArray();
        int[] next = getNextArray(str2);
        int index1 = 0;
        int index2 = 0;
        while (index1 < str1.length && index2 < str2.length) {
            if (str1[index1] == str2[index2]) {
                index1++;
                index2++;
            } else if (index2 == 0) {
                index1++;
            } else {
                index2 = next[index2];
            }
        }
        return index2 == str2.length ? index1 - index2 : -1;
    }

    public static int[] getNextArray(char[] str2) {
        if (str2.length == 1) {
            return new int[0];
        }
        int[] next = new int[str2.length];
//        next[0] = -1;
//        next[1] = 0;
        int index = 2;
        int cn = 0;
        while (index < str2.length) {
            if (str2[index - 1] == str2[cn]) {
                next[index++] = ++cn;
            } else if (cn > 0) {
                cn = next[cn];
            } else {
                next[index++] = 0;
            }
        }
        return next;
    }

    // for test
    public static String getRandomString(int possibilities, int size) {
        char[] ans = new char[(int) (Math.random() * size) + 1];
        for (int i = 0; i < ans.length; i++) {
            ans[i] = (char) ((int) (Math.random() * possibilities) + 'a');
        }
        return String.valueOf(ans);
    }

    public static void main(String[] args) {
        int possibilities = 5;
        int strSize = 20;
        int matchSize = 5;
        int testTimes = 5000000;
        System.out.println("test begin");
        for (int i = 0; i < testTimes; i++) {
            String str = getRandomString(possibilities, strSize);
            String match = getRandomString(possibilities, matchSize);
            if (getIndexOf(str, match) != str.indexOf(match)) {
                System.out.println("Oops!");
            }
        }
        System.out.println("test finish");
    }
}