KMP

package leetcode;

public class KMP {
    private int[] getNext(String needle){
        int[] next =  new int[needle.length()];
        int j = 0;
        next[0] = 0;
        for (int i = 1; i < next.length; i++) {
            while (j > 0 && needle.charAt(i) != needle.charAt(j)) {
                j = next[j - 1];
            }
            // next represent when match failed, this means needle[j] != text[i], and also means j - 1 elements equal
            // next[i] store the biggest equivalent prefix of needle[i]
            if (needle.charAt(i) == needle.charAt(j)) {
                // j point to the character to match in needle
                j++;
            }
            next[i] = j;
        }
        return next;
    }
    public int strStr(String haystack, String needle) {
        int[] next = getNext(needle);
        int j = 0;
        for (int i = 0; i < haystack.length(); i++) {
            while (j > 0 && haystack.charAt(i) != needle.charAt(j)) {
                j = next[j - 1];
            }
            if (haystack.charAt(i) == needle.charAt(j)) {
                j++;
            }
            if (j == needle.length()) {
                // when the last character(needle[needle.length() - 1]) has matched, j++, now j equals to needle.length()
                return i - j + 1;
            }

        }
        return -1;
    }
}