LeetCode-Implement strStr()-KMP

Implement strStr().

Returns the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

Update (2014-11-02):
The signature of the function had been updated to return the index instead of the pointer. If you still see your function signature returns a char * or String, please click the reload button to reset your code definition.

Solution:

public class Solution {
    public int strStr(String haystack, String needle) {
        if (needle.isEmpty()) return 0;
        if (haystack.isEmpty()) return -1;

        int[] pm = getPartialMatchTable(needle);
        int p1 = 0, p2 = 0;
        while (p1<haystack.length() && p2<needle.length()){
            if (haystack.charAt(p1)==needle.charAt(p2)){
                p1++;
                p2++;
            } else {
                if (p2==0) p1++;
                else p2 = pm[p2-1]+1;
            }
        }
        if (p2<needle.length()) return -1;
        else return p1-p2;
        
    }

    public int[] getPartialMatchTable(String needle){
        int[] pm = new int[needle.length()];
        pm[0] = -1;
        for (int i=1;i<needle.length();i++){
            int j = pm[i-1];
            while (j>=0 && needle.charAt(i)!=needle.charAt(j+1))  j = pm[j];
            if (needle.charAt(i)==needle.charAt(j+1)) pm[i] = j+1;
            else pm[i] = -1;
        }
        return pm;
    }
}