leetcode 28. Implement strStr()

问题：

Implement strStr().

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

采用KMP模式匹配算法：主要参考   http://blog.youkuaiyun.com/v_july_v/article/details/7041827

public class Solution {
    public static int[] getNext(String sub){
        int[] next=new int[sub.length()];
        int k=-1,j=0;
        next[0]=-1;
        while(j<sub.length()-1){
            if(k==-1||sub.charAt(k)==sub.charAt(j)){
                k++;
                j++;
                next[j]=k;
            }else{
                k=next[k];
            }
        }
        return next;
    }
    public int strStr(String haystack, String needle) {
        if(needle.equals(""))return 0;
        if(haystack.equals(""))return -1;
        int i=0,j=0;
        int[] next=getNext(needle);
         while(i<haystack.length()&&j<needle.length()){
             if(j==-1||haystack.charAt(i)==needle.charAt(j)){
                 i++;
                 j++;
             }else{
                j=next[j];
             }
         }
         if(j==needle.length()){
             return i-j;
         }else return -1;
    }
}