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;
     }
 }