KMP算法

public class Main {

    public static void main(String[] args) {
        String s = "abcabcababaccc";
        String m = "ab";
        
        System.out.println(getIndex(s, m));
    }

    public static int getIndex(String s, String m) {
        if(s.length() < m.length()) {
            return -1;
        }

        char[] str1 = s.toCharArray();
        char[] str2 = m.toCharArray();
        
        int i1 = 0;
        int i2 = 0;

        int[] next = getNextArray(str2);

        while(i1 < str1.length && i2 < str2.length) {
            if(str1[i1] == str2[i2]) {
                i1++;
                i2++;
            } else if(next[i2] == -1) {
                i1++;
            } else {
                i2 = next[i2];
            }
        }

        return i2 == str2.length?i1-i2:-1;
    }

    public static int[] getNextArray(char[] str) {
        if(str.length == 1) {
            return new int[]{-1};
        }

        int[] next = new int[str.length];
        next[0] = -1;
        next[1] = 0;

        int i = 2;
        int cn = 0;

        while(i < next.length) {
            if(str[i-1] == str[cn]) {
                next[i++] = ++cn;
            } else if(cn > 0) {
                cn = next[cn];
            } else {
                next[i++] = 0;
            }
        }

        return next;
    }
}