KMP算法

public class KMP {

    /**
     * Determines whether the source string and pattern string are equal.
     *
     * @param source  the source string
     * @param pattern the pattern string
     * @return Returns -1 if it does not match, and the index of the
     * first matching character in the source string if it does
     */
    public static int match(String source, String pattern) {
        if (source == null || pattern == null)
            return -1;
        if (source.length() == 0 && pattern.length() == 0)
            return -1;
        int[] next = getNext(pattern);
        int i = 0, j = 0;
        while (i < source.length() && j < pattern.length()) {
            // When j is -1, it means that we need to move
            // the main string, and j + 1 happens to be 0
            if (j == -1 || source.charAt(i) == pattern.charAt(j)) {
                i++;
                j++;
            } else {
                j = next[j];
            }
        }
        return (j == pattern.length() ? i - j : -1);
    }

    /**
     * Get the next array of the pattern string.
     *
     * @param pattern
     * @return
     */
    public static int[] getNext(String pattern) {
        if (pattern == null || pattern.length() == 0)
            return null;
        int[] next = new int[pattern.length()];
        int j = 0, k = -1;
        next[0] = -1;

        while (j < pattern.length() - 1) {
            if (k == -1 || pattern.charAt(j) == pattern.charAt(k)) {
                if (pattern.charAt(++j) == pattern.charAt(++k)) {
                    next[j] = next[k];
                } else {
                    next[j] = k;
                }
            } else {
                k = next[k];
            }
        }
        return next;
    }
}